It shouldn't make a difference for generating types and consts, but
let's use the glibc release branch anyway.

Fixes golang/go#29873

Change-Id: I15a3df250f5ae6181b2d036a10e0523ebe44ac3f
Reviewed-on: https://go-review.googlesource.com/c/158997Reviewed-by: Ian Lance Taylor <iant@golang.org>