userland: add assembly support

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

userland/arch/aarch64/c/asm_from_c.c

@@ -0,0 +1,39 @@
+/* https://github.com/cirosantilli/arm-assembly-cheat#calling-convention */
+
+#include <assert.h>
+#include <inttypes.h>
+
+uint64_t my_asm_func(void);
+/* { return 42; } */
+__asm__(
+    ".global my_asm_func;"
+    "my_asm_func:"
+    "mov x0, 42;"
+    "ret;"
+);
+
+/* Now a more complex example that also calls a C function.
+ * We have to store the return value x30 for later because bl modifies it.
+ * https://stackoverflow.com/questions/27941220/push-lr-and-pop-lr-in-arm-arch64/34504752#34504752
+ * We are not modifying any other callee saved register in this function,
+ * since my_c_func is not either (unless GCC has a bug ;-)), so everything else if fine.
+ */
+uint64_t my_asm_func_2(void);
+/* { return my_c_func(); } */
+__asm__(
+    ".global my_asm_func_2;"
+    "my_asm_func_2:"
+    "str x30, [sp, -16]!;"
+    "bl my_c_func;"
+    "ldr x30, [sp], 16;"
+    "ret;"
+);
+
+uint64_t my_c_func(void) {
+    return 42;
+}
+
+int main(void) {
+    assert(my_asm_func() == 42);
+    assert(my_asm_func_2() == 42);
+}