gdb/testsuite: XFAIL some gdb.base/fileio.exp

Some gdb.base/fileio.exp tests expect the inferior to not have write
access to some files. If the test is being run as root, this is never
possible. This commit adds a way to identify if the user is root and
xfails the tests that expect no write access.

Approved-By: Tom de Vries <tdevries@suse.de>