libstdc++: Make dg-require-namedlocale work for more targets [PR65909]

libstdc++: Make dg-require-namedlocale work for more targets [PR65909]

As noted in the PR, some embedded targets do not support command-line
arguments, which means that the dg-require-namedlocale check always
fails. Use Sandra's suggestion of hardcoding the argument into the
executable instead of passing it as a command-line argument.

Realistically, those embedded targets probably don't support the named
locales anyway, but at least now the tests will be UNSUPPORTED for the
right reason.

libstdc++-v3/ChangeLog:

	PR libstdc++/65909
	* testsuite/lib/libstdc++.exp (check_v3_target_namedlocale):
	Hardcode the locale name instead of passing it to the
	executable. Do not hardcode buffer size for string.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>