When checking undefined symbol list, ignore certain llvm buildins

These can vary between llvm version.  For example a recent upstream
change recently added __multi3 to the list:
https://reviews.llvm.org/D65143

Fixes #98

Makefile

@@ -451,12 +451,14 @@ finish: startup_files libc
 	# Collect symbol information.
 	# TODO: Use llvm-nm --extern-only instead of grep. This is blocked on
 	# LLVM PR40497, which is fixed in 9.0, but not in 8.0.
+	# Ignore certain llvm builtin symbols such as those starting with __mul
+	# since these dependencies can vary between llvm versions.
 	"$(WASM_NM)" --defined-only "$(SYSROOT_LIB)"/libc.a "$(SYSROOT_LIB)"/*.o \
 	    |grep ' [[:upper:]] ' |sed 's/.* [[:upper:]] //' |LC_ALL=C sort > "$(SYSROOT_SHARE)/defined-symbols.txt"
 	for undef_sym in $$("$(WASM_NM)" --undefined-only "$(SYSROOT_LIB)"/*.a "$(SYSROOT_LIB)"/*.o \
 	    |grep ' U ' |sed 's/.* U //' |LC_ALL=C sort |uniq); do \
 	    grep -q '\<'$$undef_sym'\>' "$(SYSROOT_SHARE)/defined-symbols.txt" || echo $$undef_sym; \
-	done > "$(SYSROOT_SHARE)/undefined-symbols.txt"
+	done | grep -v "^__mul" > "$(SYSROOT_SHARE)/undefined-symbols.txt"
 	grep '^_*wasi_' "$(SYSROOT_SHARE)/undefined-symbols.txt" \
 	    > "$(SYSROOT_LIB)/libc.imports"
 

expected/wasm32-wasi/undefined-symbols.txt

@@ -12,11 +12,6 @@ __getf2
 __gttf2
 __letf2
 __lttf2
-__muldc3
-__muloti4
-__mulsc3
-__multc3
-__multf3
 __netf2
 __stack_pointer
 __subtf3