libstdc++: Fix proc check_v3_target_namedlocale for "" locale [PR65909]

libstdc++: Fix proc check_v3_target_namedlocale for "" locale [PR65909]

When the last format argument to a Tcl proc is named 'args' it has
special meaning and is a list that accepts any number of arguments[1].
This means when "" is passed to the proc and then we expand "$args" we
get an empty list formatted as "{}". My r16-537-g3e2b83faeb6b14 change
broke all uses of dg-require-namedlocale with empty locale names, "".

By changing the name of the formal argument to 'locale' we avoid the
special behaviour for 'args' and now it only accepts a single argument
(as was always intended). When expanded as "$locale" we get "" as I
expected.

[1] https://www.tcl-lang.org/man/tcl9.0/TclCmd/proc.html

libstdc++-v3/ChangeLog:

	PR libstdc++/65909
	* testsuite/lib/libstdc++.exp (check_v3_target_namedlocale):
	Change name of formal argument to locale.

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