[memprof] dump memprof profile when receive deadly signals

Currently memprof profile is dumped when program exits (call `FinishAndWrite()` in ~Allocator) or `__memprof_profile_dump` is manually called.
For programs that never exit (e.g. server-side application), it will be useful to dump memprof profile when specific signal is received.
This patch installs a signal handler for deadly signals(SIGSEGV, SIGBUS, SIGABRT, SIGILL, SIGTRAP, SIGFPE) like we do in other sanitizers. In the signal handler `__memprof_profile_dump` is called to dump memprof profile.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D134795