Fix build breakage on FreeBSD after rL289878/rL289881

Summary:
After rL289878/rL289881, the build on FreeBSD is broken, because
sanitizer_platform_limits_posix.cc attempts to include <utmp.h> and use
`struct utmp`, neither of which are supported anymore on FreeBSD.

Fix this by adding `&& !SANITIZER_FREEBSD` in a few places, and stop
intercepting utmp functions altogether for FreeBSD.

Reviewers: kubabrecka, emaste, eugenis, ed

Subscribers: ed, llvm-commits

Differential Revision: https://reviews.llvm.org/D27902

llvm-svn: 290167