tsan: Consider SI_TIMER signals always asynchronous

POSIX timer can be configured to send any kind of signal, however, it
fundamentally does not make sense to consider a timer a synchronous
signal. Teach TSan that timers are never synchronous.

The tricky bit here is correctly defining compiler-rt's siginfo
replacement, which is a rather complex struct. Extend it in a limited
way that is mostly cross-platform compatible and add offset tests in
sanitizer_platform_limits_posix.cpp.

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D142117