Improve cosmo's conformance to libc-test

This change addresses various open source compatibility issues, so that
we pass 313/411 of the tests in https://github.com/jart/libc-test where
earlier today we were passing about 30/411 of them, due to header toil.
Please note that Glibc only passes 341/411 so 313 today is pretty good!

- Make the conformance of libc/isystem/ headers nearly perfect
- Import more of the remaining math library routines from Musl
- Fix inconsistencies with type signatures of calls like umask
- Write tests for getpriority/setpriority which work great now
- conform to `struct sockaddr *` on remaining socket functions
- Import a bunch of uninteresting stdlib functions e.g. rand48
- Introduce readdir_r, scandir, pthread_kill, sigsetjmp, etc..

Follow the instructions in our `tool/scripts/cosmocc` toolchain to run
these tests yourself. You use `make CC=cosmocc` on the test repository

libc/thread/pthread_exit.c

@@ -17,6 +17,7 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/dce.h"
+#include "libc/intrin/atomic.h"
 #include "libc/limits.h"
 #include "libc/mem/gc.h"
 #include "libc/runtime/runtime.h"
@@ -25,8 +26,6 @@
 #include "libc/thread/tls.h"
 #include "third_party/nsync/futex.internal.h"
 
-STATIC_YOINK("_pthread_main");
-
 /**
  * Terminates current POSIX thread.
  *
@@ -86,7 +85,7 @@ wontreturn void pthread_exit(void *rc) {
     // this implementation so much simpler for example we want't to call
     // set_tid_address() upon every program startup which isn't possible
     // on non-linux platforms anyway.
-    __get_tls()->tib_tid = 0;
+    atomic_store_explicit(&__get_tls()->tib_tid, 0, memory_order_release);
     nsync_futex_wake_((int *)&__get_tls()->tib_tid, INT_MAX, !IsWindows());
     _Exit1(0);
   }