userland: add assembly support

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

userland/arch/x86_64/common_arch.h

@@ -0,0 +1,84 @@
+#ifndef COMMON_ARCH_H
+#define COMMON_ARCH_H
+
+#define ASSERT_EQ(reg, const) \
+    push %rax; \
+    push %rbx; \
+    mov reg, %rax; \
+    mov const, %rbx; \
+    cmp %rax, %rbx; \
+    ASSERT(je); \
+    pop %rbx; \
+    pop %rax; \
+;
+
+# TODO
+##define ASSERT_MEMCMP(s1, s2, n) \
+#    MEMCMP(s1, s2, n); \
+#    ASSERT_EQ(x0, 0); \
+#;
+
+/* Program entry point.
+ *
+ * Return with EXIT.
+ *
+ * Basically implements an x86_64 prologue:
+ *
+ * - save callee saved registers
+ *   x86_64 explained at: https://stackoverflow.com/questions/18024672/what-registers-are-preserved-through-a-linux-x86-64-function-call/55207335#55207335
+ * - save register arguments for later usage
+ */
+#define ENTRY \
+.text; \
+.global asm_main; \
+asm_main: \
+    push %rbp; \
+    mov %rsp, %rbp; \
+    push %r15; \
+    push %r14; \
+    push %r13; \
+    push %r12; \
+    push %rbx; \
+    push %rdi; \
+    sub $8, %rsp; \
+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.
+ */
+#define EXIT \
+    mov $0, %rax; \
+    jmp pass; \
+fail: \
+    mov -0x8(%rbp), %rbx; \
+    movl %eax, (%rbx); \
+    mov $1, %rax; \
+pass: \
+    pop %rbx; \
+    pop %r12; \
+    pop %r13; \
+    pop %r14; \
+    pop %r15; \
+    add $16, %rsp; \
+    pop %rbp; \
+    ret; \
+;
+
+#define FAIL \
+    mov $__LINE__, %eax; \
+    jmp fail; \
+;
+
+# TODO
+##define MEMCMP(s1, s2, n) \
+#    adr x0, s1; \
+#    adr x1, s2; \
+#    ldr x2, =n; \
+#    bl memcmp; \
+#;
+
+#endif