Auto merge of #713 - marmistrz:fix-gettid, r=alexcrichton

Bring back SYS_gettid on Android/ARM which was accidentally removed by #710.

src/unix/notbsd/android/b32/arm.rs

@@ -5,3 +5,5 @@ pub const O_DIRECT: ::c_int = 0x10000;
 pub const O_DIRECTORY: ::c_int = 0x4000;
 pub const O_NOFOLLOW: ::c_int = 0x8000;
 pub const O_LARGEFILE: ::c_int = 0o400000;
+
+pub const SYS_gettid: ::c_long = 224;