sanitizer_common: fix SIG_DFL warning

Currently we have:

sanitizer_posix_libcdep.cpp:146:27: warning: cast between incompatible
  function types from ‘__sighandler_t’ {aka ‘void (*)(int)’} to ‘sa_sigaction_t’
  146 |     sigact.sa_sigaction = (sa_sigaction_t)SIG_DFL;

We don't set SA_SIGINFO, so we need to assign to sa_handler.
And SIG_DFL is meant for sa_handler, so this gets rid of both
compiler warning, type cast and potential runtime misbehavior.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D102162