[llvm] [BOLT][AArch64] Add base_cflags for PI flags (PR #145502)

[Paschalis Mpeis via llvm-commits]

paschalis-mpeis wrote:

> I'd vote for making it explicit: pie/nopie. If you have a different opinion, let's check with others: cc @maksfb @yota9

I also prefer making everything explicit. This change would require replacing every `%cflags` / `%cxxflags` with `pie_*` or `nopie_*` variants.

Uploaded a **draft** patch to see if is what we want. If we agree, I'll follow-up with a full patch.

https://github.com/llvm/llvm-project/pull/145502