Add support for openat on x86_64

arch/x86_64.c

@@ -20,7 +20,7 @@ intptr_t write(int32_t fd, const void* buf, intptr_t size) {
       "movq %0, %%rax\n"
       : "=r"(n_written)
       : "a"(SYS_WRITE), "D"(fd), "S"(buf), "d"(size)  // RDI, RSI, RDX are used in x86-64 for write args
-      : "rcx", "r11", "memory"
+      : "rcx", "r11"
   );
   return n_written;
 }
@@ -50,4 +50,18 @@ uint32_t fork() {
   );
   return (uint32_t)rtn;
 }
+
+#define SYS_OPENAT 257
+int32_t openat(int32_t fd, const char* filename, uint32_t flags, uint32_t mode) {
+  int32_t file;
+  asm volatile (
+      "mov %%r10, %5\n"
+      "syscall\n"
+      : "=a"(file)
+      : "a"(SYS_OPENAT), "D"(fd), "S"(filename), "d"(flags), "r"((uint64_t)mode)  // RDI, RSI, RDX are used in x86-64 for write args
+      : "rcx", "r10", "r11"
+  );
+  return file;
+}
+
 #endif /* ifdef __x86_64__ */