[libunwind] [libcxx][libcxxabi][libunwind] Support target-specific flags for tests (PR #65517)

[Louis Dionne via cfe-commits]

ldionne wrote:

The direction we've been taking with these config files is to keep the base config setup as simple as possible and avoid the proliferation of options that are not fundamentally tied to the test suite. These kinds of options should instead be hardcoded in a suitable `.cfg.in` file for the platform you're trying to test. So instead of trying to set `-mabi=` here, you should create a `riscv` config file that describes how to build and run the test suite on that platform. This constrains the platform complexity to a single file and avoids leaking into the rest of the test suite.

I'll tentatively close this for now based on the above reasoning, but please reopen for discussion if you disagree.

https://github.com/llvm/llvm-project/pull/65517