threads.h: avoid defining gettid on glibc >= 2.30

The issue in https://bugs.chromium.org/p/chromium/issues/detail?id=1182060 also
exists on glibc 2.30 and 2.31 since `gettid` was Introduced in glibc 2.30.

See https://man7.org/linux/man-pages/man2/gettid.2.html

Bug: 285204695
Test: Build
Change-Id: I7e534edf8c0a20c415232bcfffabbf2c1d6eec98