Update chpl_mem to default to dlmalloc or none when target arch is knc.

This is motivated by the fact that tcmalloc does not configure or build well in a knc environment. For
one, it tries to compile an then run some c++ code, which doesn't work with a knc compiler. There is
an option to cross compile by passing `--host=<some host string>` to `./configure`, but Ben and I ran
into other issues when we tried that. We decided that for now, it is best to not use tcmalloc
when compiling knc.

I worked with @benharsh on this fix.