powerpc: enable supported sanitizers on powerpc64*

[pkubaj]

Summary

1. Merge LLVM's 315d792130258a9b7250494be8d002ebb427b08f, adding support

for FreeBSD/powerpc64*.

2. Add sanitizer list to lib/libclang_rt/Makefile, taken from the list of

libraries that llvm-devel port builds.

3. powerpc64le supports the same sanitizers that powerpc64, but powerpc64le

also supports xray* sanitizers.

4. lib/libclang_rt/xray/Makefile hardcodes amd64-specific files, so that needs

to be conditionalized.

5. Sanitizers are not enabled for powerpc, because powerpc supports only

builtins and profile.

Test Plan

make buildworld on powerpc64*

[dim]

LGTM