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

hildeth/chapel,chizarlicious/chapel,hildeth/chapel,chizarlicious/chapel,chizarlicious/chapel,chizarlicious/chapel,CoryMcCartan/chapel,CoryMcCartan/chapel,CoryMcCartan/chapel,chizarlicious/chapel,hildeth/chapel,chizarlicious/chapel,hildeth/chapel,hildeth/chapel,CoryMcCartan/chapel,chizarlicious/chapel,hildeth/chapel,hildeth/chapel,CoryMcCartan/chapel,CoryMcCartan/chapel,CoryMcCartan/chapel