userland: add assembly support

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

userland/arch/arm/freestanding/build

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

userland/arch/arm/freestanding/hello.S

@@ -0,0 +1,21 @@
+/* https://github.com/cirosantilli/arm-assembly-cheat#linux-system-calls */
+
+.syntax unified
+.text
+.global _start
+_start:
+asm_main_after_prologue:
+    /* write */
+    mov r0, 1     /* stdout */
+    adr r1, msg   /* buffer */
+    ldr r2, =len  /* len */
+    mov r7, 4     /* syscall number */
+    svc 0
+
+    /* exit */
+    mov r0, 0     /* exit status */
+    mov r7, 1     /* syscall number */
+    svc 0
+msg:
+    .ascii "hello\n"
+len = . - msg