Do not use array parameter to new_composite_name (bug 26726)

Among the warnings causing a glibc build with GCC 11 to fail is one
for a call new_composite_name in setlocale.c.  The newnames argument
is declared as an array with __LC_LAST elements, but when the category
argument is not LC_ALL, it actually only has one element.  Since the
number of elements depends on the first argument to the function, it
seems clearer to declare the argument as a pointer.

Tested with build-many-glibcs.py for arm-linux-gnueabi, where this
allows the build to get further.

Reviewed-by: DJ Delorie <dj@redhat.com>