fix crashing sigsetjmp on s390x

the bz instruction that was wrongly used only admits a small immediate
displacement and cannot be used with external symbols; apparently the
linker fails to diagnose the overflow.

src/signal/s390x/sigsetjmp.s

@@ -6,7 +6,7 @@
 sigsetjmp:
 __sigsetjmp:
 	ltgr  %r3, %r3
-	bz    ___setjmp
+	jz    ___setjmp
 
 	stg   %r14, 18*8(%r2)
 	stg   %r6,  20*8(%r2)