Use Linux 5.1 in build-many-glibcs.py [committed]

[Joseph Myers]

I've now reverted this patch.

Linux 5.1 headers are not in fact usable for glibc testing, because
"[PATCH] uapi: avoid namespace conflict in linux/posix_types.h"
<https://lore.kernel.org/lkml/20190319165123.3967889-1-arnd@arndb.de/>
did not get merged for 5.1 and so many conform/ tests fail.

We'll need to await a fix for that issue reaching a Linux kernel release, 
or the issue otherwise somehow being worked around in glibc, before we can 
use kernel headers newer than 5.0.

-- 
Joseph S. Myers
joseph@codesourcery.com