Package the rest of the locales as glibc-locales

• Use provided target to install C.UTF-8 locale
  The previous code used localedef from lib32 and was also missing I18NPATH.

  The build system already has targets for installing locales, but the split build makes invoking them a bit more difficult.

  This installs the locale as C.utf8, which glibc seems to prefer.

• Use DESTDIR instead of install_root
  Both work but DESTDIR is less weird.

• Improve shell hygiene

  • Rename skip_test() to _skip_test()
  • Use subshell to prevent LDFLAGS change from leaking out of check()
  • Use subshells to prevent environment leaks from builds

• Package the rest of the locales as glibc-locales
  These are useful for checkdepends. Use hardlink to find duplicate files, which reduces the installed size from 1007 MB to 234 MB.

  See: archlinux/devtools#90