[libcxx] [libcxxabi] [libunwind] [llvm] [runtimes] remove workaround for old CMake when setting `--unwindlib=none` (PR #93429)

[via cfe-commits]

h-vetinari wrote:

So I've been trying to follow down the rabbit hole of the failing flag checks, and it seems the combination of `CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG` plus https://gitlab.kitware.com/cmake/cmake/-/issues/23454 has a wider blast radius than anticipated.

I'm not claiming that adding the `_previous_CMAKE_{REQUIRED_LINK_OPTIONS,TRY_COMPILE_TARGET_TYPE}` dance everywhere is the right approach here, but it was - so far - the obvious path to just try to get things green again. It's conceivable though that it would be easier to simply shift the detection of `CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG` until after the other flag checks have been performed? 🤔 

https://github.com/llvm/llvm-project/pull/93429