userland: add assembly support

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

userland/arch/arm/str.S

@@ -0,0 +1,60 @@
+/* https://github.com/cirosantilli/arm-assembly-cheat#load-and-store-instructions */
+
+#include "common.h"
+
+.data;
+    /* Must be in the .data section, since we want to modify it. */
+myvar:
+    .word 0x12345678
+
+ENTRY
+    /* r0 will contain the address. */
+    ldr r0, =myvar
+
+    /* Sanity check. */
+    ldr r1, [r0]
+    movw r2, 0x5678
+    movt r2, 0x1234
+    ASSERT_EQ_REG(r1, r2)
+
+    /* Modify the value. */
+    movw r1, 0xDEF0
+    movt r1, 0x9ABC
+    str r1, [r0]
+
+    /* Check that it changed. */
+    ldr r1, [r0]
+    movw r2, 0xDEF0
+    movt r2, 0x9ABC
+    ASSERT_EQ_REG(r1, r2)
+
+    /* Cannot use PC relative addressing to a different segment,
+     * or else it fails with:
+     *
+     * ....
+     * Error: internal_relocation (type: OFFSET_IMM) not fixed up
+     * ....
+     *
+     * https://stackoverflow.com/questions/10094282/internal-relocation-not-fixed-up
+     */
+    /*ldr r0, myvar*/
+
+#if 0
+    /* We could in theory write this to set the address of myvar,
+     * but it will always segfault under Linux because the text segment is read-only.
+     * This is however useful in baremetal programming.
+     * This construct is not possible in ARMv8 for str:
+     * https://github.com/cirosantilli/arm-assembly-cheat#armv8-str
+     */
+    str r1, var_in_same_section
+var_in_same_section:
+#endif
+
+    /* = sign just doesn't make sense for str, you can't set the
+     * address of a variable.
+     */
+#if 0
+    str r1, =myvar
+#endif
+
+EXIT