[compiler-rt] Don't explictly ad-hoc sign dylibs on APPLE if ld is new enough

ld64 implicitly ad-hoc code-signs as of Xcode 12, and `strip` and friends know
how keep this special ad-hoc signature valid.

So this should have no effective behavior change, except that you can now strip
libclang_rt.asan_osx_dynamic.dylib and it'll still have a valid ad-hoc
signature, instead of strip printing "warning: changes being made to the file
will invalidate the code signature in:" and making the ad-hoc code signature
invalid.

Differential Revision: https://reviews.llvm.org/D123475