build: depends move macOS C(XX) FLAGS out of C & CXX
Summary:
Move some C/CXX FLAGS out of C/CXX. The remaining flags are host/SDK related, and will need some more thought. This is more correct in any case, and simplifies future changes.
Backport of core#29233.
Depends on D17249.
Test Plan: Run the macos guix build.
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D17250