compiler-rt: add aarch64 init function for LSE atomics

As reported by Ronald, adding the out-of-line LSE atomics helpers for
aarch64 to compiler-rt was not sufficient to link programs using these,
as they also require a __aarch64_have_lse_atomics global. This is
initialized in compiler-rt's lib/builtins/cpu_model.c, roughly similar
to the x86 CPU model and feature detection in that file.

Since upstream does not yet have a FreeBSD specific implementation for
getting the required information, add a simple one that should work for
now, while I try to get it sorted with the LLVM people.

Reported by: Ronald Klop <ronald-lists@klop.ws>
Fixes: cc55ee8009a5
PR: 257392
MFC after: 2 weeks