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

[antoine moynault via cfe-commits]

antmox wrote:

I may be missing something, but it looks like the armv7m-picolibc-no-exceptions issue is still related to https://gitlab.kitware.com/cmake/cmake/-/issues/23454

I notice that once the `--unwindlib=none` flag is added to `CMAKE_REQUIRED_LINK_OPTIONS`, it is also added to ar commands during the next cmake tests (starting with `CXX_SUPPORTS_NOSTDLIBXX_FLAG`).
Failing these tests will result in `fno-exceptions` not being used later

This is probably due to the fact that for Armv7M-picolibc, `CMAKE_TRY_COMPILE_TARGET_TYPE` is set to `STATIC_LIBRARY` here: https://github.com/llvm/llvm-project/blob/main/libcxx/cmake/caches/Armv7M-picolibc.cmake#L7C35-L7C49


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