Fix abort(3) to raise SIGABRT rather than causing SIGSEGV.

tgkill() needs the .save stack unwinding directive to get the complete
stack trace.

BUG: https://code.google.com/p/android/issues/detail?id=16672

Change-Id: Ifb447dca2147a592c48baf32769dfc175d8aea72

Conflicts:
	libc/unistd/abort.c