allow get_dr_mcontext() to be called from dr_init and primary thread init

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on April 26, 2013 12:01:27_

xref issue #117 which added mcontext for all but primary thread
xref issue #626 to have mcontext not point at return of our routine

It would simplify clients to make the initial app mcontext available at
init time. Currently clients like Dr. Memory work around this by waiting
for the 1st bb.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1152_