Implement the "abort" stub in assembly for ARM.

So that we can always get the full stack trace regardless of gcc's handling
of the "noreturn" attribute associated with abort().

[cherry-picked from master]

BUG:6455193
Change-Id: I0102355f5bf20e636d3feab9d1424495f38e39e2

libc/unistd/abort.c

@@ -102,29 +102,3 @@ abort(void)
 	(void)kill(getpid(), SIGABRT);
 	_exit(1);
 }
-
-#ifdef __arm__
-/*
- * abort() does not return, which gcc interprets to mean that it doesn't
- * need to preserve any of the callee-save registers.  Unfortunately this
- * includes the link register, so if LR is used there is no way to determine
- * which function called abort().
- *
- * We work around this by inserting a trivial stub that doesn't alter
- * any of the "interesting" registers and thus doesn't need to save them.
- * We can't just call __libc_android_abort from C because gcc uses "bl"
- * without first saving LR, so we use an asm statement.  This also has
- * the side-effect of replacing abort() with __libc_android_abort() in
- * the stack trace.
- *
- * Ideally __libc_android_abort would be static, but I haven't figured out
- * how to tell gcc to call a static function from an asm statement.
- */
-void
-abort(void)
-{
-    asm ("b __libc_android_abort");
-    _exit(1);       /* suppress gcc noreturn warnings */
-}
-#endif
-