userland: add assembly support

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

userland/arch/aarch64/c/reg_var_float.c

@@ -0,0 +1,28 @@
+/* https://github.com/cirosantilli/arm-assembly-cheat#register-variables */
+
+#include <assert.h>
+#include <inttypes.h>
+
+int main(void) {
+    register double d0 __asm__ ("d0");
+    register double d1 __asm__ ("d1");
+    double new_d0;
+    double new_d1;
+    {
+        d0 = 1.5;
+        d1 = 2.5;
+        __asm__ (
+            "fmov d2, 1.5;"
+            "fadd %d[d0], d0, d2;"
+            "fadd %d[d1], d1, d2;"
+            : [d0] "+w" (d0),
+              [d1] "+w" (d1)
+            :
+            : "d2"
+        );
+        new_d0 = d0;
+        new_d1 = d1;
+    }
+    assert(new_d0 == 3.0);
+    assert(new_d1 == 4.0);
+}