[OpenMP] libomptarget: move debugging dumps under control of env var LIBOMPTARGET_DEBUG

Disable default debugging dumps for libomptarget and plugins and move dumps
under control of environment variable LIBOMPTARGET_DEBUG=<integer>. Dumps
are enabled when LIBOMPTARGET_DEBUG is set to a positive integer value.

Debugging dumps are available only in debug build; release build does not
support it.

Differential Revision: https://reviews.llvm.org/D33227

llvm-svn: 310841