[OpenMP][OMPT] Introduce VERBOSE_INIT in ompt-multiplex.h

OpenMP 5.1 added OMP_TOOL_VERBOSE_INIT. This env variable is
extremely helpful to understand the issue when loading a tool fails
unexpectedly (e.g., errors from dlopen, when the libc available at
runtime is older than libc used at compile time of the tool -> missed
to load the right gcc module).

This patch replicates the verbose init code from libomp watching
out for a different env variable. Similar to
CLIENT_TOOL_LIBRARIES_VAR, a tool can define the name of
the env var by defining CLIENT_TOOL_VERBOSE_INIT_VAR
before including ompt-multiplex.h.
Alternatively, a tool can define OMPT_MULTIPLEX_TOOL_NAME
to specify the tool name which will be the prefix for both
_TOOL_LIBRARIES and _VERBOSE_INIT var.
Finally, if none of the two macros is defined, the header will
print a compiler warning and look at OMP_TOOL_VERBOSE_INIT.

Patch prepared by Semih Burak

Differential Revision: https://reviews.llvm.org/D112809