remove possible-textrels from powerpc asm

these are perfectly fine with ld-time symbol binding, but otherwise
result in textrels. they cannot be replaced with @PLT jump targets
because the PLT thunks require a GOT register to be setup, so use a
hidden alias instead.

src/setjmp/powerpc/setjmp.s

+	.global ___setjmp
+	.hidden ___setjmp
 	.global __setjmp
 	.global _setjmp
 	.global setjmp
 	.type   __setjmp,@function
 	.type   _setjmp,@function
 	.type   setjmp,@function
+___setjmp:
 __setjmp:
 _setjmp:
 setjmp:

src/signal/powerpc/sigsetjmp.s

@@ -12,7 +12,8 @@ __sigsetjmp:
 	stw 16, 448+4+8(3)
 	mr 16, 3
 
-	bl setjmp
+.hidden ___setjmp
+	bl ___setjmp
 
 	mr 4, 3
 	mr 3, 16
@@ -23,4 +24,4 @@ __sigsetjmp:
 .hidden __sigsetjmp_tail
 	b __sigsetjmp_tail
 
-1:	b setjmp
+1:	b ___setjmp