[clang] [libcxx] [libcxxabi] [Fuchsia][cmake] Allow using FatLTO when building runtimes (PR #112277)

[Petr Hosek via cfe-commits]

petrhosek wrote:

I think this is due to https://github.com/llvm/llvm-project/blob/c3edeaa61bf0e7faed6c26f693b4fcd9102ba1ec/compiler-rt/lib/builtins/cpu_model/aarch64/lse_atomics/fuchsia.inc#L7 which I believe is yet another instance of #56070.

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