compiler-rt: re-exec with ASLR disabled when necessary

Some sanitizers (at least msan) currently require ASLR to be disabled.
When we detect that ASLR is enabled, re-exec with it disabled rather
than exiting with an error. See LLVM GitHub issue 53256 for more
detail: https://github.com/llvm/llvm-project/issues/53256

No objection: dim
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33934

(cherry picked from commit 930a7c2ac67e1e8e511aa1d0a31a16c632060ebb)