compiler-rt: Use FreeBSD's elf_aux_info to detect AArch64 HW features

Using the out-of-line LSE atomics helpers for AArch64 on FreeBSD also
requires adding support for initializing __aarch64_have_lse_atomics
correctly. On Linux this is done with getauxval(3), on FreeBSD with
elf_aux_info(3), which has a slightly different interface.

Differential Revision: https://reviews.llvm.org/D109330