Fix some bugs in OMPT support

Fix some bugs in OMPT support

1.) in kmp_csupport.c, move computation of parameters only needed for OMPT tracing
inside a conditional to reduce overhead if not receiving ompt_event_master_begin
callbacks.
2.) in kmp_gsupport.c, remove spurious reset of OMPT reenter_runtime_frame (which
is set in its caller, GOMP_parallel_start correct placement of #if OMP_TRACE so
that state is maintained even if tracing support not included.
3.) in z_Linux_util.c, add architecture independent support for OMPT by setting
and resetting OMPT's exit_frame_ptr before and after invoking a microtask.
4.) On the Intel MIC, the loader refuses to retain static symbols in the
libomp.so shared library, even though tools need them. The loader could not be
bullied into doing so. To accommodate this, I changed the visibility of OMPT
placeholder functions to public. This required additions in exports.so.txt,
adding extern "C" scoping in ompt-general.c so that the public placeholder
symbols won't be mangled.

Patch by John Mellor-Crummey

Differential Revision: http://reviews.llvm.org/D11062

llvm-svn: 242052