threads.h: avoid defining gettid on glibc >= 2.32

Glibc >=2.32 exposes a gettid() which clashes with libcutils
thread.h, so add a check to not expose it if building against
newer glibc (ChromiumOS will still use glibc 2.27 besides 2.32).

Bug: https://bugs.chromium.org/p/chromium/issues/detail?id=1182060
Test: Builds without errors on both glibc 2.32 and 2.27.

Change-Id: Ib71fa1bc9fa185e3668002407dbed05a80c87740