[clang] [libcxx] [libcxxabi] [libcxx][cmake] Allow using FatLTO in libc++ builds (PR #112277)

[Paul Kirth via cfe-commits]

ilovepi wrote:

Oh, I finally see what you mean. In the per target runtimes, none of the top level flags get propagated unless you specify them w/ the correct target prefix. I was so confused about why things worked this way.

https://github.com/llvm/llvm-project/pull/112277