gdb/testsuite: remove XFAIL on gdb.cp/temargs.exp

gdb.cp/temargs.exp last 2 tests always setup an XFAILs, despite checking
for old gcc versions.  However, Clang does not fail in this test,
turning into XPASSes and slighty annoying when comparing between
compilers.  To change this, make the xfails only happen if we using gcc.

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