[MSan] Fix wrong unpoison size in SignalAction

MSan should unpoison the paramters of extended signal handlers.
However, MSan unpoisoned the second parameter with size
`sizeof(__sanitizer_sigaction)`, inconsistent with its real type
`siginfo_t`.

This commit fix this issue by correcting the size to
`sizeof(__sanitizer_siginfo)`..

Author: Camsyn

compiler-rt/lib/msan/msan_interceptors.cpp

@@ -1127,7 +1127,7 @@ static void SignalAction(int signo, void *si, void *uc) {
   SignalHandlerScope signal_handler_scope;
   ScopedThreadLocalStateBackup stlsb;
   UnpoisonParam(3);
-  __msan_unpoison(si, sizeof(__sanitizer_sigaction));
+  __msan_unpoison(si, sizeof(__sanitizer_siginfo));
   __msan_unpoison(uc, ucontext_t_sz(uc));
 
   typedef void (*sigaction_cb)(int, void *, void *);