Rollup merge of #149633 - tgross35:freebsd-outline-atomics, r=davidtwco

Enable `outline-atomics` by default on AArch64 FreeBSD

Many aarch64 targets without LSE in the baseline enable the
`outline-atomics` feature, which uses runtime detection of LSE for its
faster atomic ops. This provides nontrivial performance improvements on
most hardware from the past decade, at a small cost to anything pre-LSE.

This matches what Clang does [1].

Context: rust-lang/rust#144938

[1]: llvm/llvm-project@e24f901

try-job: dist-x86_64-freebsd

Author: matthiaskrgr