Do not link asan_rtl_x86_64.S for non x86_64 platforms.

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 | https://bugs.chromium.org/p/chromium/issues/detail?id=1427165 ]]

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D149957