Support use of LLVM's libunwind for exception unwinding

It is built in libgcc_s.so and libgcc_eh.a to simplify transition.

It is enabled by default on arm64 (where we previously had no other
unwinder) and may be enabled for testing on other platforms by setting
WITH_LLVM_LIBUNWIND in src.conf(5).

Also add compiler-rt's __gcc_personality_v0 implementation for use with
the LLVM unwinder.

Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D4787