[lld-macho] Allow linking with ABI compatible architectures

Linking fails when targeting `x86_64-apple-darwin` for runtimes. The issue
is that LLD strictly assumes the target architecture be present in the tbd
files (which isn't always true). For example, when targeting `x86_64h`, it should
work with `x86_64` because they are ABI compatible. This is also inline with what
ld64 does.

An environment variable (which ld64 also supports) is also added to preserve the
existing behavior of strict architecture matching.

Reviewed By: #lld-macho, int3

Differential Revision: https://reviews.llvm.org/D130683