[OpenACC 'kernels'] '-fopenacc-kernels=[...]' -> '--param=openacc-kernels=[...]'

This configuration knob is temporary, and isn't really meant to be exposed to
users.

	gcc/
	* params.opt (-param=openacc-kernels=): Add.
	* omp-oacc-kernels-decompose.cc
	(pass_omp_oacc_kernels_decompose::gate): Use it.
	* doc/invoke.texi (-fopenacc-kernels=@var{mode}): Move...
	(--param): ... here, 'openacc-kernels'.
	gcc/c-family/
	* c.opt (fopenacc-kernels=): Remove.
	gcc/fortran/
	* lang.opt (fopenacc-kernels=): Remove.
	gcc/testsuite/
	* c-c++-common/goacc/if-clause-2.c: '-fopenacc-kernels=[...]' ->
	'--param=openacc-kernels=[...]'.
	* c-c++-common/goacc/kernels-decompose-1.c: Likewise.
	* c-c++-common/goacc/kernels-decompose-2.c: Likewise.
	* c-c++-common/goacc/kernels-decompose-ice-1.c: Likewise.
	* c-c++-common/goacc/kernels-decompose-ice-2.c: Likewise.
	* gfortran.dg/goacc/kernels-decompose-1.f95: Likewise.
	* gfortran.dg/goacc/kernels-decompose-2.f95: Likewise.
	* gfortran.dg/goacc/kernels-tree.f95: Likewise.
	libgomp/
	* testsuite/libgomp.oacc-c-c++-common/declare-vla-kernels-decompose-ice-1.c:
	'-fopenacc-kernels=[...]' -> '--param=openacc-kernels=[...]'.
	* testsuite/libgomp.oacc-c-c++-common/declare-vla-kernels-decompose.c:
	Likewise.
	* testsuite/libgomp.oacc-c-c++-common/kernels-decompose-1.c:
	Likewise.
	* testsuite/libgomp.oacc-fortran/pr94358-1.f90: Likewise.