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