[openmp] Workaround for HSA in issue 60119

Move plugin initialization to libomptarget initialization.

Removes the call_once control, probably fractionally faster overall.
Fixes issue 60119 because the plugin initialization, which might
try to dlopen unrelated shared libraries, is no longer nested within
a call from application code.

Fixes #60119

Reviewed By: Maetveis, jhuber6

Differential Revision: https://reviews.llvm.org/D142249