userland: add assembly support

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

userland/arch/arm/ldr_pseudo.S

@@ -0,0 +1,65 @@
+/* https://github.com/cirosantilli/arm-assembly-cheat#ldr-pseudo-instruction */
+
+#include "common.h"
+
+ENTRY
+
+    /* Mnemonic for a PC relative load:
+     *
+     * ....
+     * ldr r0, [pc, offset]
+     * r0 = myvar
+     * ....
+     */
+    ldr r0, myvar
+    ASSERT_EQ(r0, 0x12345678)
+
+    /* Mnemonic PC relative load with an offset.
+     * Load myvar2 instead of myvar.
+     */
+    ldr r0, myvar + 4
+    ASSERT_EQ(r0, 0x9ABCDEF0)
+
+    /* First store the address in r0 using a magic =myvar, which creates
+     * a new variable containing the address and PC-relative addresses it
+     * https://stackoverflow.com/questions/17214962/what-is-the-difference-between-label-equals-sign-and-label-brackets-in-ar
+     *
+     * Use the adr instruction would likely be better for this application however.
+     *
+     * ....
+     * r0 = &myvar
+     * r1 = *r0
+     * ....
+     */
+    ldr r0, =myvar
+    ldr r1, [r0]
+    ASSERT_EQ(r1, 0x12345678)
+
+    /* More efficiently, use r0 as the address to read, and write to r0 itself. */
+    ldr r0, =myvar
+    ldr r0, [r0]
+    ASSERT_EQ(r0, 0x12345678)
+
+    /* Same as =myvar but store a constant to a register.
+     * Can also be done with movw and movt. */
+    ldr r0, =0x11112222
+    ASSERT_EQ(r0, 0x11112222)
+
+    /* We can also use GAS tolower16 and topper16  and movw and movt
+     * to load the address of myvar into r0 with two immediates.
+     *
+     * This results in one extra 4 byte instruction read from memory,
+     * and one less data read, so it is likely more cache efficient.
+     *
+     * https://sourceware.org/binutils/docs-2.19/as/ARM_002dRelocations.html
+     */
+    movw r0, #:lower16:myvar
+    movt r0, #:upper16:myvar
+    ldr r1, [r0]
+    ASSERT_EQ(r1, 0x12345678)
+
+EXIT
+myvar:
+    .word 0x12345678
+myvar2:
+    .word 0x9ABCDEF0