[sanitizer] Improve FreeBSD ASLR detection

The kern.elf64.aslr.pie_enable and kern.elf32.aslr.pie_enable sysctls
control the default setting for PIE binary address randomization, but
it is possible to enable or disable ASLR on a per-process basis.  So,
use procctl(2) to query whether ASLR is enabled.

(Note that with ASLR enabled but sysctl kern.elf64.aslr.pie_enable=0
a PIE binary will in effect have randomization disabled, and would be
functional with msan.  This is not intended as as a user-facing control
though; proccontrol(1) should be used to disable aslr for the process.)

Reviewed By: devnexen

Differential Revision: https://reviews.llvm.org/D117521