[gdb/testsuite] Improve xfail in gdb.cp/nsusing.exp

[gdb/testsuite] Improve xfail in gdb.cp/nsusing.exp

In test-case gdb.cp/nsusing.exp I came across these xfails without PRMS
mentioned:
...
XFAIL: gdb.cp/nsusing.exp: print x, before using statement
XFAIL: gdb.cp/nsusing.exp: print x, only using M
...

Add the missing PRMS, such that we have:
...
XFAIL: gdb.cp/nsusing.exp: print x, before using statement (PRMS gcc/108716)
XFAIL: gdb.cp/nsusing.exp: print x, only using M (PRMS gcc/108716)
...
and limit the xfail to unfixed versions.

The PR is fixed starting gcc 13, but it has been backported to release
branches stretching back to gcc 10.  For simplicity we just stick to testing
for the major version and ignore the backported fixes.

Tested on x86_64-linux.

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