Auto merge of rust-lang#3079 - madsmtm:pthread-main-apple, r=JohnTitor

Add `pthread_main_np` on Apple targets

Available since macOS 10.4 and iOS 2.0 ([source](https://github.com/apple-oss-distributions/libpthread/blob/67e155c94093be9a204b69637d198eceff2c7c46/include/pthread/pthread.h#L537-L539)).

Author: bors

libc-test/semver/apple.txt

@@ -2100,6 +2100,7 @@ pthread_mutexattr_setpshared
 pthread_rwlockattr_getpshared
 pthread_rwlockattr_setpshared
 pthread_setname_np
+pthread_main_np
 ptrace
 pututxline
 pwritev

src/unix/bsd/apple/mod.rs

@@ -5197,6 +5197,7 @@ extern "C" {
         attr: *const pthread_condattr_t,
         pshared: *mut ::c_int,
     ) -> ::c_int;
+    pub fn pthread_main_np() -> ::c_int;
     pub fn pthread_mutexattr_setpshared(
         attr: *mut pthread_mutexattr_t,
         pshared: ::c_int,