kbuild: add cmd_file_size

Some architectures support self-extracting kernel, which embeds the
compressed vmlinux.

It has 4 byte data at the end so the decompressor can know the vmlinux
size beforehand.

GZIP natively has it in the trailer, but for the other compression
algorithms, the hand-crafted trailer is added.

It is unneeded to generate such _corrupted_ compressed files because
it is possible to pass the size data as a separate file.

For example, the assembly code:

     .incbin "compressed-vmlinux-with-size-data"

can be transformed to:

     .incbin "compressed-vmlinux"
     .incbin "size-data"

My hope is, after some reworks of the decompressors, the macros
cmd_{bzip2,lzma,lzo,lz4,xzkern,zstd22}_with_size will go away.

This new macro, cmd_file_size, will be useful to generate a separate
size-data file.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>