Userland+Tests: Don't use MAP_FILE when mmap-ing

MAP_FILE is not in POSIX, and is simply in most LibCs as a "default"
mode. Our own LibC defines it as 0, meaning "no flags". It is also not
defined in some OS's, such as Haiku. Let's be more portable and not use
the unnecessary flag.

Tests/Kernel/mmap-write-into-running-programs-executable-file.cpp

@@ -18,7 +18,7 @@ int main()
         perror("open");
         return 1;
     }
-    u8* ptr = (u8*)mmap(nullptr, 16384, PROT_READ, MAP_FILE | MAP_SHARED, fd, 0);
+    u8* ptr = (u8*)mmap(nullptr, 16384, PROT_READ, MAP_SHARED, fd, 0);
     if (ptr == MAP_FAILED) {
         perror("mmap");
         return 1;