LibC: Make "errno" thread-specific

Now that the kernel supports thread-local storage, we can declare errno
with the __thread keyword, which causes it to be per-thread.

This should fix all the stupid issues that happen when many threads use
the same errno. :^)

Libraries/LibC/crt0.cpp

@@ -6,7 +6,7 @@ extern "C" {
 
 int main(int, char**);
 
-int errno;
+__thread int errno;
 char** environ;
 bool __environ_is_malloced;