Add generic C.UTF-8 locale (Bug 17318)

Add generic C.UTF-8 locale (Bug 17318)

We add a new C.UTF-8 locale.  This locale is not builtin to glibc, but
is provided as a distinct locale.  The locale provides full support
for UTF-8 and this includes full code point sorting via strcmp-based
collation.

The collation uses a new keyword 'strcmp_collation' which drops all
collation rules and generates an empty zero rules collation to enable
strcmp usage in collation. This ensures that we get full code point
sorting for C.UTF-8 with a minimal 92 bytes of overhead (LC_COLLATE
structure information).

The new locale is added to SUPPORTED.  Minimal test data for specific
code points (minus those not supported by collate-test) is provided
in C.UTF-8.in, and this verifies code point sorting is working
reasonably across the range.  The locale was tested manually with the
full set of code points without failure.

The locale is harmonized with locales already shipping in Gentoo,
Debian, Ubuntu, Fedora, CentOS Stream, and RHEL. A new tst-iconv9 test
is added which verifies the C.UTF-8 locale is generally usable.

Testing for fnmatch, regexec, and recomp is provided by extending
bug-regex1, bugregex19, bug-regex4, bug-regex6, transbug, tst-fnmatch,
tst-regcomp-truncated, and tst-regex to use C.UTF-8.

Tested on x86_64 or i686 without regression.