[contrib][linux-kernel] Generate SPDX license identifiers (#3294)

[contrib][linux-kernel] Generate SPDX license identifiers (#3294)

Add a `--spdx` option to the freestanding script to prefix
files with a line like (for `.c` files):

    // SPDX-License-Identifier: GPL-2.0+ OR BSD-3-Clause

or (for `.h` and `.S` files):

    /* SPDX-License-Identifier: GPL-2.0+ OR BSD-3-Clause */

Given the style of the line to be used depends on the extension,
a simple `sed` insert command would not work.

It also skips the file if an existing SPDX line is there,
as well as raising an error if an unexpected SPDX line appears
anywhere else in the file, as well as for unexpected
file extensions.

I double-checked that all currently generated files appear
to be license as expected with:

    grep -LRF 'This source code is licensed under both the BSD-style license (found in the'  linux/lib/zstd
    grep -LRF 'LICENSE file in the root directory of this source tree) and the GPLv2 (found' linux/lib/zstd

but somebody knowledgable on the licensing of the project should
double-check this is the intended case.

Fixes: https://github.com/facebook/zstd/issues/3293
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>