userland: add assembly support

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

userland/arch/aarch64/c/multiline.cpp

@@ -0,0 +1,18 @@
+// https://stackoverflow.com/questions/3666013/how-to-write-multiline-inline-assembly-code-in-gcc-c/54575948#54575948
+
+#include <assert.h>
+#include <inttypes.h>
+
+int main(void) {
+    uint64_t io = 0;
+    __asm__ (
+        R"(
+add %[io], %[io], #1
+add %[io], %[io], #1
+)"
+        : [io] "+r" (io)
+        :
+        :
+    );
+    assert(io == 2);
+}