posix-arm: new directory, with tas.c from Skip Tavakkolian

R=rsc
http://codereview.appspot.com/6408043

posix-arm/tas.c

@@ -0,0 +1,35 @@
+#include "u.h"
+#include "libc.h"
+
+int
+tas(long *x)
+{
+	int     v, t, i = 1;
+
+#if ARMv5
+	__asm__(
+		"swp  %0, %1, [%2]"
+		: "=&r" (v)
+		: "r" (1), "r" (x)
+		: "memory"
+	);
+#else
+	__asm__ (
+		"1:	ldrex	%0, [%2]\n"
+		"	strex	%1, %3, [%2]\n"
+		"	teq	%1, #0\n"
+		"	bne	1b"
+		: "=&r" (v), "=&r" (t)
+		: "r" (x), "r" (i)
+		: "cc");
+#endif
+	switch(v) {
+	case 0:
+	case 1:
+		return v;
+	default:
+		print("canlock: corrupted 0x%lux\n", v);
+		return 1;
+	}
+}
+