gdb/linux-tdep: use builtin integer types

I noticed some spots creating new integer types, where I think we could
use the builtin types created by create_gdbtypes_data instead.

The old code to initialize `short_type` was actually passing the wrong
bit length value, so I think this actually fixes a latent bug, although
it goes away in a subsequent patch.

Change-Id: I8fc71cb75f4450a42f48ba2af5c1163208e58d8c
Approved-By: Tom Tromey <tom@tromey.com>