libroot/musl: Import m68k atomic_arch.h.

Change-Id: I33aef6b417177ac7b998715b60ced7f28be8c3eb
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6908
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

src/system/libroot/posix/musl/arch/m68k/atomic_arch.h

@@ -0,0 +1,8 @@
+#define a_cas a_cas
+static inline int a_cas(volatile int *p, int t, int s)
+{
+	__asm__ __volatile__ (
+		"cas.l %0, %2, (%1)"
+		: "+d"(t) : "a"(p), "d"(s) : "memory", "cc");
+	return t;
+}