Fix chpl_gmp.py to support new install path.
Fix chpl_gmp.py to support new install path.

The install path changed in e7c4859. This supports the new version.

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