Introduce #include <cosmo.h> to toolchain users

This change improves the way internal APIs are being hidden behind the
`COSMO` define. The cosmo.h header will take care of defining that, so
that a separate define statement isn't needed. This change also does a
lot more to define which APIs are standard, and which belong to Cosmo.

libc/thread/pthread_attr_init.c

@@ -27,7 +27,7 @@
 errno_t pthread_attr_init(pthread_attr_t *attr) {
   *attr = (pthread_attr_t){
       .__stacksize = GetStackSize(),
-      .__guardsize = GUARDSIZE,
+      .__guardsize = APE_GUARDSIZE,
   };
   return 0;
 }