This API wraps conventional gettid() linux-specific API, which even in
Linux itself is sometimes not properly supported/announced.
This API also allows future porting to other platforms if needed, and so
far falls back to getpid() if no gettid(9 can be found.
Code ported from osmo-trx.git, see commit 7a07de1efd4eb7cc11c33d3ad25cb2df70aa1ef1.
Related: OS#5027
Change-Id: Id7534beeb22fcd50813dab76dd68818e2ff87ec2