Allow for early calls to chpl_task_getRequestedSubloc().

[Reviewed by sungeun.]

We sometimes need to call chpl_task_getRequestedSubloc() prior to the
point at which the tasking layer has been initialized.  Add support for
this.


git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@21902 3a8e244f-b0f2-452b-bcba-4c88e055c3ca