userland: add assembly support

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

userland/arch/x86_64/c/scratch_hardcode.c

@@ -0,0 +1,20 @@
+/* This is a worse version of scratch.c with hardcoded scratch. */
+
+#include <assert.h>
+#include <inttypes.h>
+
+int main(void) {
+    uint64_t in1 = 0xFFFFFFFF;
+    uint64_t in2 = 1;
+    uint64_t out;
+    __asm__ (
+        "mov %[in2], %%rax;" /* scratch = in2 */
+        "add %[in1], %%rax;" /* scratch += in1 */
+        "mov %%rax, %[out];" /* out = scratch */
+        : [out] "=r" (out)
+        : [in1] "r" (in1),
+          [in2] "r" (in2)
+        : "rax"
+    );
+    assert(out == 0x100000000);
+}