fix(sys-libs/glibc): Build only en_US locale by default.

This is only applicable to building the SDK (boards ship with only the
built-in C and POSIX locales). The SDK chroot sets en_US by default so
there is no reason to waste time building other locales for the tarball.
Merge pull request #445 from marineam/locale

fix(sys-libs/glibc): Build only en_US locale by default.