fix sigsetjmp on arm (needs asm)

no idea why gcc refuses to compile the C code to use a tail call, but
it's best to use asm anyway so we don't have to rely on the quality of
the compiler's optimizations for correct code.

src/signal/arm/sigsetjmp.s

@@ -0,0 +1,13 @@
+.global sigsetjmp
+.type sigsetjmp,%function
+sigsetjmp:
+	str a2,[a1,#256]
+	tst a2,a2
+	beq setjmp
+	push {a1,lr}
+	add a3,a1,#260
+	mov a2,#0
+	mov a1,#2
+	bl sigprocmask
+	pop {a1,lr}
+	b setjmp