If bundled gmp is built, even on a cray, default to CHPL_GMP=gmp.

Motivated by knc, but will be useful when/if bundled gmp is built for
cray-x*. For knc, bundled gmp is the only supported option (aside from
CHPL_GMP=none).
Merge pull request #255 from thomasvandoren/update-chplenv-for-knc

If bundled gmp is built, even on a cray, default to CHPL_GMP=gmp.

reviewed by @sungeunchoi