backport 18efa420 - compiler-rt: Allow build without __c11_atomic_fetch_nand

[brooksdavis]

Please backport 18efa42 to fix FreeBSD builds with older compilers (e.g., clang 13) where a toolchain isn't bootstrapped.

[tstellar]

/cherry-pick 18efa42

[llvmbot]

/branch llvm/llvm-project-release-prs/issue55813

[llvmbot]

/pull-request llvm/llvm-project-release-prs#7

[tstellar]

@efriedma-quic What do you think about backporting this?