arm: Provide alloca()

This provides a simple implementation of alloca for ARM (and enables
the associated testcase).  As tcc for ARM doesn't contain an assembler,
we'll have to resort using gcc for compiling it.

lib/alloca-arm.S

@@ -0,0 +1,11 @@
+	.text
+	.align	2
+	.global	alloca
+	.type	alloca, %function
+alloca:
+	rsb	sp, r0, sp
+	bic	sp, sp, #7
+	mov	r0, sp
+	mov	pc, lr
+	.size	alloca, .-alloca
+	.section	.note.GNU-stack,"",%progbits