[sanitizer] remove FreeBSD PS_STRINGS fallback

The PS_STRINGS constant can easily be incorrect with mismatched
kernel/userland - e.g. when building i386 sanitizers on FreeBSD/amd64
with -m32. The kern.ps_strings sysctl was introduced over 20 years ago
as the supported way to fetch the environment and argument string
addresses from the kernel, so the fallback is never used.

Differential Revision:	http://reviews.llvm.org/D19027

llvm-svn: 266305