Add gettid() for Linux and Android

Fixes #2076

src/unix/linux_like/android/mod.rs

@@ -2809,6 +2809,8 @@ extern "C" {
     pub fn regfree(preg: *mut ::regex_t);
 
     pub fn android_set_abort_message(msg: *const ::c_char);
+
+    pub fn gettid() -> ::pid_t;
 }
 
 cfg_if! {

src/unix/linux_like/linux/mod.rs

@@ -3594,6 +3594,8 @@ extern "C" {
         outbytesleft: *mut ::size_t,
     ) -> ::size_t;
     pub fn iconv_close(cd: iconv_t) -> ::c_int;
+
+    pub fn gettid() -> ::pid_t;
 }
 
 cfg_if! {