Rewrite init app in C, introduce MSL (MOP3 System Library)

libmsl/amd64/syscall.c

@@ -0,0 +1,23 @@
+#include <stddef.h>
+#include <stdint.h>
+#include <amd64/syscall.h>
+
+int msl_amd64_syscall (int syscall_num, uintptr_t a1, uintptr_t a2, uintptr_t a3, uintptr_t a4, uintptr_t a5, uintptr_t a6) {
+  uint64_t result;
+  __asm__ volatile (
+    "movq %1, %%rax\n"
+    "movq %2, %%rdi\n"
+    "movq %3, %%rsi\n"
+    "movq %4, %%rdx\n"
+    "movq %5, %%r10\n"
+    "movq %6, %%r8\n"
+    "movq %7, %%r9\n"
+    "syscall\n"
+    "movq %%rax, %0\n"
+    : "=r"(result)
+    : "r"((uint64_t)syscall_num), "r"(a1), "r"(a2), "r"(a3), "r"(a4), "r"(a5), "r"(a6)
+    : "memory", "cc", "rcx", "r11"
+  );
+  return (int)result;
+}
+