[MC][NFC] Ensure tests that intended to cover compact unwind code paths do so

These two tests were intended to cover the CU code paths, but invoked
llvm-mc using a generic "darwin" platform. However, MC only attempts to
emit CU for `macos >= 10.6` [1]. Thus we adjust the tests accordingly.

compact-unwind-cfi_def_cfa.s was added in 2fca51d3
fp-setup-macho.s was added in 03ffa797

[1]: https://github.com/llvm/llvm-project/blob/16c93aaa4a955676fe05f876bd036719a1b14b2d/llvm/lib/MC/MCObjectFileInfo.cpp#L43

Reviewed By: MaskRay, smeenai

Differential Revision: https://reviews.llvm.org/D124126