userland: add assembly support

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

userland/arch/aarch64/movk.S

@@ -0,0 +1,26 @@
+/* https://github.com/cirosantilli/arm-assembly-cheat#movk */
+
+#include "common.h"
+
+ENTRY
+    movk x0, 0x4444, lsl 0
+    movk x0, 0x3333, lsl 16
+    movk x0, 0x2222, lsl 32
+    movk x0, 0x1111, lsl 48
+    ASSERT_EQ(x0, 0x1111222233334444)
+
+    /* Set a label (addresses are 48-bit) with immediates:
+     *
+     * * https://stackoverflow.com/questions/38570495/aarch64-relocation-prefixes
+     * * https://sourceware.org/binutils/docs-2.26/as/AArch64_002dRelocations.html
+     *
+     * This could be used if the label is too far away for
+     * adr relative addressing.
+     */
+    movz x0, :abs_g2:label     /* bits 32-47, overflow check */
+    movk x0, :abs_g1_nc:label  /* bits 16-31, no overflow check */
+    movk x0, :abs_g0_nc:label  /* bits  0-15, no overflow check */
+    adr x1, label
+label:
+    ASSERT_EQ_REG(x0, x1)
+EXIT