userland: add assembly support

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

userland/arch/aarch64/freestanding/build

@@ -0,0 +1 @@
+../build

userland/arch/aarch64/freestanding/hello.S

@@ -0,0 +1,20 @@
+/* https://github.com/cirosantilli/arm-assembly-cheat#linux-system-calls */
+
+.text
+.global _start
+_start:
+asm_main_after_prologue:
+    /* write */
+    mov x0, 1     /* stdout */
+    adr x1, msg   /* buffer */
+    ldr x2, =len  /* len */
+    mov x8, 64    /* syscall number */
+    svc 0
+
+    /* exit */
+    mov x0, 0     /* exit status */
+    mov x8, 93    /* syscall number */
+    svc 0
+msg:
+    .ascii "hello\n"
+len = . - msg