[esan] Ensure internal_sigaction() bypasses interceptors

Summary:
Implements real_sigaction() which it turns out is required for
internal_sigaction() to bypass the libc interceptors.

Without real_sigaction(), our internal_sigaction() calls during init happen
to work due to the EsanDuringInit check in COMMON_INTERCEPTOR_ENTER (though
even here it does not feel right for an "internal_" call to go through the
interceptor).  The real problem is when we call internal_sigaction() after
we're initialized, which only happens on an unhandled SIGSEGV for which the
app has no handler: then we'll spin in an infinite loop as our attempts to
remove our own handler repeatedly fail.  It's not easy to add a test for
that, unfortunately.

Reviewers: aizatsky

Subscribers: vitalybuka, zhaoqin, kcc, eugenis, llvm-commits, kubabrecka

Differential Revision: http://reviews.llvm.org/D20832

llvm-svn: 271626