libcpp: Add support for gnu::offset #embed/__has_embed parameter

libcpp: Add support for gnu::offset #embed/__has_embed parameter

The following patch adds on top of the just posted #embed patch
a first extension, gnu::offset which allows to seek in the data
file (for seekable files, otherwise read and throw away).
I think this is useful e.g. when some binary data start with
some well known header which shouldn't be included in the data etc.

2024-09-12  Jakub Jelinek  <jakub@redhat.com>

libcpp/
	* internal.h (struct cpp_embed_params): Add offset member.
	* directives.cc (EMBED_PARAMS): Add gnu::offset entry.
	(enum embed_param_kind): Add NUM_EMBED_STD_PARAMS.
	(_cpp_parse_embed_params): Use NUM_EMBED_STD_PARAMS rather than
	NUM_EMBED_PARAMS when parsing standard parameters.  Parse gnu::offset
	parameter.
	* files.cc (struct _cpp_file): Add offset member.
	(_cpp_stack_embed): Handle params->offset.
gcc/
	* doc/cpp.texi (Binary Resource Inclusion): Document gnu::offset
	#embed parameter.
gcc/testsuite/
	* c-c++-common/cpp/embed-15.c: New test.
	* c-c++-common/cpp/embed-16.c: New test.
	* gcc.dg/cpp/embed-5.c: New test.