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.