Do not link asan_rtl_x86_64.S for non x86_64 platforms.

[CaseySmalley]

Summary

Do not link asan_rtl_x86_64.S for non x86_64 platforms.

Prior to this CL asan_rtl_x86_64.S would be always be linked when
building compiler-rt for non x86_64 platforms, this normally isn't an
issue since at that link time is an empty file anyway.

When attempting to link a Aarch64 program with branch protection enabled
with the address sanitizer.

E.g.

clang --target=aarch64 \
      -fsanitize=address \
      -mbranch-protection=standard \
      -Wl,-z,force-bti
      -o test \
      test.cc

Results in the linking error from the sections generated from the empty
asan_rtl_x86_64.S owed to missing a .note.gnu.property

Also see https://bugs.chromium.org/p/chromium/issues/detail?id=1427165

[MaskRay]

compiler-rt/lib/asan/CMakeLists.txt

49

On x86-64, if the host compiler supports -m32, default builds will support both i386 and x86_64. This condition will not build asan_rtl_x86_64.S.

I have fixed this in 5717236f7e1fde7948569466fa7dfd7c6562f160

compiler-rt/lib/asan/CMakeLists.txt

 set(ASAN_CXX_SOURCES
   asan_new_delete.cpp
   )
 
 set(ASAN_STATIC_SOURCES
   asan_rtl_static.cpp
   )
 
-if (NOT WIN32 AND NOT APPLE)
+if (ASAN_SUPPORTED_ARCH STREQUAL "x86_64" AND NOT WIN32 AND NOT APPLE)
   list(APPEND ASAN_STATIC_SOURCES
     asan_rtl_x86_64.S
   )
 endif()
 
 set(ASAN_PREINIT_SOURCES
   asan_preinit.cpp
   )