ARC: make sigaction inline with other arches

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
ARC: clone: Ensure that @fn returns back to clone

This showed up due to longstanding test/unistd/clone failure where post
clone, the callback was not exiting and rather falling thru in into main
program.

STAR ARS0095996

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

Conflicts:
	libc/sysdeps/linux/arc/clone.S