add pthread_equal

(backport <#3773>)
(cherry picked from commit 83b3393)

Author: joboet

libc-test/semver/unix.txt

@@ -687,6 +687,7 @@ pthread_condattr_init
 pthread_condattr_t
 pthread_create
 pthread_detach
+pthread_equal
 pthread_exit
 pthread_getspecific
 pthread_join

src/unix/mod.rs

@@ -1056,6 +1056,7 @@ extern "C" {
     pub fn times(buf: *mut ::tms) -> ::clock_t;
 
     pub fn pthread_self() -> ::pthread_t;
+    pub fn pthread_equal(t1: ::pthread_t, t2: ::pthread_t) -> ::c_int;
     #[cfg_attr(
         all(target_os = "macos", target_arch = "x86"),
         link_name = "pthread_join$UNIX2003"