Parallelize more third-party makefiles

Add new variable to `Makefile.base`, `CHPL_MAKE_NPROCS`, which is suitable to pass to -j argument
when calling make. Use this variable in makefiles under third-party/ to parallelize builds.

gmp and llvm makefiles already calculated number of procs and set -j with that value. They now use
the new variable.

The main motivation here is to speedup the build process for developers and users. tcmalloc, hwloc,
and qthreads are built by default and can take quite a while. re2 and gmp will also be built by default
soon. This should help keep the built time for third-party/ at a minimum. (It will also reduce the time it
takes travis to run the smoke tests.)