userland: add assembly support

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

userland/arch/arm/common_arch.h

@@ -0,0 +1,71 @@
+#ifndef COMMON_ARCH_H
+#define COMMON_ARCH_H
+
+.syntax unified
+
+/* Assert that a register equals a constant.
+ * * reg: the register to check. Can be r0-r10, but not r11. r11 is overwritten.
+ * * const: the constant to compare to. Only works for literals or labels, not for registers.
+ *          For register / register comparision, use ASSERT_EQ_REG.
+ */
+#define ASSERT_EQ(reg, const) \
+    ldr r11, =const; \
+	cmp reg, r11; \
+	ASSERT(beq); \
+;
+
+/* Assert that two arrays are the same. */
+#define ASSERT_MEMCMP(s1, s2, n) \
+	MEMCMP(s1, s2, n); \
+	ASSERT_EQ(r0, 0); \
+;
+
+/* Store all callee saved registers, and LR in case we make further BL calls.
+ *
+ * Also save the input arguments r0-r3 on the stack, so we can access them later on,
+ * despite those registers being overwritten.
+ */
+#define ENTRY \
+.text; \
+.global asm_main; \
+asm_main: \
+    stmdb sp!, {r0-r12, lr}; \
+asm_main_after_prologue: \
+;
+
+/* Meant to be called at the end of ENTRY.*
+ *
+ * Branching to "fail" makes tests fail with exit status 1.
+ *
+ * If EXIT is reached, the program ends successfully.
+ *
+ * Restore LR and bx jump to it to return from asm_main.
+ */
+#define EXIT \
+    mov r0, 0; \
+    mov r1, 0; \
+    b pass; \
+fail: \
+    ldr r1, [sp]; \
+    str r0, [r1]; \
+    mov r0, 1; \
+pass: \
+    add sp, 16; \
+    ldmia sp!, {r4-r12, lr}; \
+    bx lr; \
+;
+
+/* Always fail. */
+#define FAIL \
+    ldr r0, =__LINE__; \
+    b fail; \
+;
+
+#define MEMCMP(s1, s2, n) \
+    ldr r0, =s1; \
+    ldr r1, =s2; \
+    ldr r2, =n; \
+    bl memcmp; \
+;
+
+#endif