Fix wasm include paths, unbreaking compiler-rt builds on !x86

compiler-rt WebAssembly builds were failing, and as a workaround commit
ca9dbba introduced COMPILER_RT_WASM_ENABLE, disabling its compilation on
several architectures (arm64 armel mips64el mipsel ppc64el armhf).

This was ultimately caused by the WebAssembly driver including paths in
bare /usr/include, when compiling for wasnNN-unknown-unknown targets.
This in turn resulted in this chain of include paths when building
compiler-rt, as one example out of many:
  1. compiler-rt/lib/builtins/divtf3.c:15                  #include "fp_lib.h"
  2. compiler-rt/lib/builtins/fp_lib.h:23                  #include "int_lib.h"
  3. compiler-rt/lib/builtins/int_lib.h:93                 #include <limits.h>
  4. /usr/lib/llvm-14/lib/clang/14.0.6/include/limits.h:20 #if __has_include_next(<limits.h>) #include_next <limits.h>
  5. /usr/include/limits.h:26                              #include <bits/libc-header-start.h>
  6. fatal error: 'bits/libc-header-start.h' file not found

/usr/include/stdint.h is the host's glibc header, and
bits/libc-header-start.h doesn't exist for the wasm targets. This is the
case with or without wasi-libc, as wasi-libc is musl-based and doesn't
have these paths at all.

The only reason builds worked on amd64 is accident:lly the B-D chain
brings in libc6-dev-i386 (through g++-multilib), which creates this
symlink:
  /usr/include/bits -> x86_64-linux-gnu/bits
This effectively meant that on amd64 builds, compiler-rt for wasm
targets was compiled with glibc x86_64 headers.

Ultimately this was rooted on the sysroot-based assumptions that the
upstream driver makes and that we are patching (evidently incompletely)
with our wasm-sysroot-usr.diff patch.

Update our patch to explicitly NOT include the bare /usr/include path on
non-OS targets (wasm32-unknown-unknown etc.), while keeping existing
behavior for backwards compatibility when --sysroot is passed.

Given this should (fingers crossed) address this invariance between
amd64 and other architectures, and unbreak those builds, revert commit
ca9dbba and the COMPILER_RT_WASM_ENABLE flag.

debian/patches/wasm/wasm-sysroot-usr.diff

@@ -27,21 +27,31 @@ Index: llvm-toolchain-14-14.0.6/clang/lib/Driver/ToolChains/WebAssembly.cpp
  
    if (!DriverArgs.hasArg(options::OPT_nobuiltininc)) {
      SmallString<128> P(D.ResourceDir);
-@@ -421,10 +423,10 @@ void WebAssembly::AddClangSystemIncludeA
+@@ -419,12 +421,20 @@ void WebAssembly::AddClangSystemIncludeA
+     return;
+   }
  
++  // add the multiarch path on e.g. wasm32-wasi
    if (getTriple().getOS() != llvm::Triple::UnknownOS) {
      const std::string MultiarchTriple =
 -        getMultiarchTriple(D, getTriple(), D.SysRoot);
 -    addSystemInclude(DriverArgs, CC1Args, D.SysRoot + "/include/" + MultiarchTriple);
 +        getMultiarchTriple(D, getTriple(), SysRoot);
++    addSystemInclude(DriverArgs, CC1Args, SysRoot + "/local/include/" + MultiarchTriple);
++    addSystemInclude(DriverArgs, CC1Args, SysRoot + "/local/include");
 +    addSystemInclude(DriverArgs, CC1Args, SysRoot + "/include/" + MultiarchTriple);
    }
 -  addSystemInclude(DriverArgs, CC1Args, D.SysRoot + "/include");
-+  addSystemInclude(DriverArgs, CC1Args, SysRoot + "/include");
++
++  // also add the non-multiarch path, only on a known OS (as above), or when
++  // a sysroot is given, for backwards compatibility with the original driver
++  if (getTriple().getOS() != llvm::Triple::UnknownOS ||
++      !getDriver().SysRoot.empty())
++    addSystemInclude(DriverArgs, CC1Args, SysRoot + "/include");
  }
  
  void WebAssembly::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
-@@ -467,3 +469,15 @@ SanitizerMask WebAssembly::getSupportedS
+@@ -467,3 +477,15 @@ SanitizerMask WebAssembly::getSupportedS
  Tool *WebAssembly::buildLinker() const {
    return new tools::wasm::Linker(*this);
  }

debian/rules

@@ -302,12 +302,6 @@ ifeq ($(LIBUNWIND_ENABLE),yes)
 endif
 endif
 
-COMPILER_RT_WASM_ENABLE=yes
-
-ifneq (,$(filter $(DEB_HOST_ARCH), arm64 armel mips64el mipsel ppc64el armhf))
-	COMPILER_RT_WASM_ENABLE=no
-endif
-
 # Do not install objects
 STAGE_ALL_CMAKE_EXTRA += -DMLIR_INSTALL_AGGREGATE_OBJECTS=OFF
 
@@ -716,7 +710,6 @@ debian-libclc-build:
 	touch $@
 
 debian-rtlib-wasm-build:
-ifeq (${COMPILER_RT_WASM_ENABLE},yes)
 	echo "Using cmake: $(CMAKE_BIN)"
 	for build in wasm32 wasm64; do \
 		mkdir -p build-compiler-rt-$$build; \
@@ -746,9 +739,6 @@ ifeq (${COMPILER_RT_WASM_ENABLE},yes)
 		-DCOMPILER_RT_OS_DIR=wasi; \
 		ninja -C build-compiler-rt-$$build $(NJOBS) $(VERBOSE); \
 	done
-else
-	echo "Skip on this arch"
-endif
 	touch $@
 
 
@@ -901,10 +891,9 @@ endif
 	$(DEB_INST)/usr/lib/llvm-$(LLVM_VERSION)/build/utils/lit/lit/*/__pycache__/
 
 	DESTDIR=$(DEB_INST) ninja $(VERBOSE) -C libclc/build install
-ifeq (${COMPILER_RT_WASM_ENABLE},yes)
+
 	DESTDIR=$(DEB_INST) ninja $(VERBOSE) -C build-compiler-rt-wasm32 install
 	DESTDIR=$(DEB_INST) ninja $(VERBOSE) -C build-compiler-rt-wasm64 install
-endif
 
 # Rename binaries
 	mkdir -p $(DEB_INST)/usr/bin/