userland: add assembly support

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

userland/arch/aarch64/pc.S

@@ -0,0 +1,78 @@
+/* https://github.com/cirosantilli/arm-assembly-cheat#registers */
+
+#include "common.h"
+
+ENTRY
+#if 0
+    /* Unlike v7, we can't use PC like any other register in ARMv8,
+     * since it is not a general purpose register anymore.
+     *
+     * Only branch instructions can modify the PC.
+     *
+     * B1.2.1 "Registers in AArch64 state" says:
+     *
+     * Software cannot write directly to the PC. It
+     * can only be updated on a branch, exception entry or
+     * exception return.
+     */
+    ldr pc, =10f
+    FAIL
+10:
+#endif
+#if 0
+    mov x0, pc
+#endif
+
+    /* LDR PC-relative loads exist in ARMv8, but they have a separate encoding
+     * "LDR (literal)" instead of "LDR (immediate)":
+     * https://stackoverflow.com/questions/28638981/howto-write-pc-relative-adressing-on-arm-asm/54480999#54480999
+     */
+    ldr x0, pc_relative_ldr
+    b 1f
+pc_relative_ldr:
+    .quad 0x123456789ABCDEF0
+1:
+    ASSERT_EQ(x0, 0x123456789ABCDEF0)
+
+    /* Just for fun, we can also use relative numbers instead of labels.
+     * https://reverseengineering.stackexchange.com/questions/17666/how-does-the-ldr-instruction-work-on-arm/20567#20567
+     */
+    ldr x0, 0x8
+    b 1f
+    .quad 0x123456789ABCDEF0
+1:
+    ASSERT_EQ(x0, 0x123456789ABCDEF0)
+
+    /* Analogous for b with PC. */
+    mov x0, 0
+    /* Jumps over mov to ASSERT_EQ. */
+    b 8
+    mov x0, 1
+    ASSERT_EQ(x0, 0)
+
+    /* Trying to use the old "LDR (immediate)" PC-relative
+     * syntax does not work.
+     */
+#if 0
+    /* 64-bit integer or SP register expected at operand 2 -- `ldr x0,[pc]' */
+    ldr x0, [pc]
+#endif
+
+    /* There is however no analogue for str. TODO rationale? */
+#if 0
+    /* Error: invalid addressing mode at operand 2 -- `str x0,pc_relative_str' */
+    str x0, pc_relative_str
+#endif
+
+    /* You just have to use adr + "STR (register)". */
+    ldr x0, pc_relative_str
+    ASSERT_EQ(x0, 0x0)
+    adr x1, pc_relative_str
+    ldr x0, pc_relative_ldr
+    str x0, [x1]
+    ldr x0, pc_relative_str
+    ASSERT_EQ(x0, 0x123456789ABCDEF0)
+EXIT
+.data
+pc_relative_str:
+    .quad 0x0000000000000000