https://bugs.llvm.org/show_bug.cgi?id=32379
(Closes: #876072)
* Fix sanitizer build failure with glibc-2.26.
* build using gold on arm64 and s390x. For backports, arm64 might still
need the BFD linker, and building with only one or two processes in
parallel.
* On amd64, s390x, arm64 and ppc64el, build with -g1 instead of -g.
* Set CMAKE_CXX_FLAGS_RELWITHDEBINFO and pass opt_flags.
