gport: Fix build with gcc 4.8 and clang

thread_local is a reserved word in gcc 4.8 and clang.
Don't use it as a variable name.

Change-Id: Ia3e6a539ab741ac0f5383c3f6113611a25c6c0de
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>