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

[Paschalis Mpeis via llvm-commits]

paschalis-mpeis wrote:

Yep, it's included by default in `%cflags`/`%cxxflags`. I'd also prefer to be explicit, but I understand why it's the default.

As an alternative to `%base_cflags`, we could introduce `%pie_cflags` and `%nopie_cflags` (likewise for cxx).
What do you think?

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