[OMPT] Use alloca() to force availability of frame pointer

When compiling with icc, there is a problem with reenter frame addresses in
parallel_begin callbacks in the interoperability.c testcase. (The address is
not available. thus NULL)
Using alloca() forces availability of the frame pointer.

Patch provided by Simon Convent

Differential Revision: https://reviews.llvm.org/D48282

llvm-svn: 336088