[OMPT] Cleanup reset of exit_frame pointer

This is done at call-site and does not need to be handled in
__kmp_invoke_microtask. It was already absent from the x86
and x86_64 assembly, this patch removes it from the generic
implementation in z_Linux_util.cpp and adds documentation for
AArch64 and PPC64 that it's actually not needed. I can't test
on these architectures, so I don't want to change the code just
because it looks right :)

While at it, rename some variables for consistency and add a
check in test/ompt/parallel/normal.c that the pointer was reset
before entering the barrier.

Differential Revision: https://reviews.llvm.org/D64442

llvm-svn: 366721