userland: add assembly support

Move arm assembly cheat here, and start some work on x86 cheat as well.

userland/arch/x86_64/c/scratch.c

@@ -0,0 +1,22 @@
+/* https://stackoverflow.com/questions/6682733/gcc-prohibit-use-of-some-registers/54963829#54963829 */
+
+#include <assert.h>
+#include <inttypes.h>
+
+int main(void) {
+    uint64_t in1 = 0xFFFFFFFF;
+    uint64_t in2 = 1;
+    uint64_t out;
+    uint64_t scratch;
+    __asm__ (
+        "mov %[in2], %[scratch];" /* scratch = in2 */
+        "add %[in1], %[scratch];" /* scratch += in1 */
+        "mov %[scratch], %[out];" /* out = scratch */
+        : [scratch] "=&r" (scratch),
+          [out] "=r" (out)
+        : [in1] "r" (in1),
+          [in2] "r" (in2)
+        :
+    );
+    assert(out == 0x100000000);
+}