TargetParser: fix getProcessTriple in universal builds

The bug happens when you build e.g. an x64_64;arm64 JIT with
LLVM_HOST_TRIPLE=x86_64-apple-macos, and then run it on an apple-m1 not under
Rosetta. In that case, sys::getProcessTriple() will return an x86_64 triple,
not an arm64 one.

Differential revision: https://reviews.llvm.org/D138449