[gdb/testsuite] Fix gdb.arch/i386-pkru.exp for native-gdbserver

With test-case gdb.arch/i386-pkru.exp and target board native-gdbserver we run
into:
...
FAIL: gdb.arch/i386-pkru.exp: variable after reading pkru
...

This looks similar to the the problem for which there's already an xfail, so
fix this by extending the xfail matching.

Tested on x86_64-linux.

Also tested on openSUSE Tumbleweed, where all tests in the test-case pass.