Don't throw --static in start_test for perf tests on mac

[reviewed by kyle]

--static isn't supported on darwin so throwing it in start_test just causes the
compiler to print out warnings that screw up testing results/validation. This
just changes start_test so we only add --static to perf runs not on darwin
systems.


git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23807 3a8e244f-b0f2-452b-bcba-4c88e055c3ca