arm: avoid conditional branch to PLT in sigsetjmp

The R_ARM_THM_JUMP19 relocation type generated for the original code
when targeting Thumb 2 is not supported by the gold linker.

src/signal/arm/sigsetjmp.s

@@ -6,9 +6,10 @@
 sigsetjmp:
 __sigsetjmp:
 	tst r1,r1
-	beq setjmp
+	bne 1f
+	b setjmp
 
-	str lr,[r0,#256]
+1:	str lr,[r0,#256]
 	str r4,[r0,#260+8]
 	mov r4,r0