Parallelize make calls when building GMP.

Building GMP, especially with the check target, takes quite a while on our
systems. It works with the -j flag, though. That is now enabled similar to llvm
where the argument is the cpu count.

r: @vasslitvinov

git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23808 3a8e244f-b0f2-452b-bcba-4c88e055c3ca