[PowerPC] Save a dword in the powerpc64 signal trampoline

In r291668, an instruction was added to sigcode64.S without the nop pad at
the end being taken out.

Due to alignment, this means that a dword is being wasted on the shared
page for no reason.

Take out this nop, and add some comments while I'm here.

Reviewed by:	jhibbits
Sponsored by:	Tag1 Consulting, Inc.
Differential Revision:	https://reviews.freebsd.org/D23055

sys/powerpc/powerpc/sigcode64.S

@@ -63,9 +63,13 @@ CNAME(sigcode64_elfv2):
 	addi	3,1,112+SF_UC		/* restore sp, and get &frame->sf_uc */
 	li	0,SYS_sigreturn
 	sc				/* sigreturn(scp) */
+	/*
+	 * If we get back to here, it means sigreturn failed.
+	 * As such, we are now stuck in the wrong context.
+	 * Exit immediately without touching the stack.
+	 */
 	li	0,SYS_exit
 	sc				/* exit(errno) */
-	nop				/* align to doubleword */
 endsigcode64:
 	
 	.data