GTY: Explicitly reject 'string_length' option for (fields in) global variables

GTY: Explicitly reject 'string_length' option for (fields in) global variables

This is preparational for another thing that I'm working on.  No change in
behavior -- other than a more explicit error message.

The 'string_length' option currently is not supported for (fields in) global
variables.  For example, if we apply the following (made-up) changes:

    --- gcc/c-family/c-cppbuiltin.cc
    +++ gcc/c-family/c-cppbuiltin.cc
    @@ -1777 +1777 @@ struct GTY(()) lazy_hex_fp_value_struct
    -  const char *hex_str;
    +  const char * GTY((string_length("strlen(%h.hex_str) + 1"))) hex_str;

    --- gcc/varasm.cc
    +++ gcc/varasm.cc
    @@ -66 +66 @@ along with GCC; see the file COPYING3.  If not see
    -extern GTY(()) const char *first_global_object_name;
    +extern GTY((string_length("strlen(%h.first_global_object_name) + 1"))) const char *first_global_object_name;

..., we get:

    [...]
    build/gengtype  \
                        -S [...]/source-gcc/gcc -I gtyp-input.list -w tmp-gtype.state
    /bin/sh [...]/source-gcc/gcc/../move-if-change tmp-gtype.state gtype.state
    build/gengtype  \
                        -r gtype.state
    [...]/source-gcc/gcc/varasm.cc:66: global `first_global_object_name' has unknown option `string_length'
    [...]/source-gcc/gcc/c-family/c-cppbuiltin.cc:1789: field `hex_str' of global `lazy_hex_fp_values[0]' has unknown option `string_length'
    make[2]: *** [Makefile:2890: s-gtype] Error 1
    [...]

These errors occur when writing "GC roots", where -- per my understanding --
'string_length' isn't relevant for actual GC purposes.  However, like
elsewhere, it is for PCH purposes, and simply accepting 'string_length' here
isn't sufficient: we'll still get '(gt_pointer_walker) &gt_pch_n_S' used in the
'struct ggc_root_tab' instances, and there's no easy way to change that to
instead use 'gt_pch_n_S2' with explicit 'size_t string_len' argument.  (At
least not sufficiently easy to justify spending any further time on, given that
I don't have an actual use for that feature.)

So, until an actual need arises, and/or to avoid the next person looking into
this having to figure out the same thing again, let's just document this
limitation:

    [...]/source-gcc/gcc/varasm.cc:66: option `string_length' not supported for global `first_global_object_name'
    [...]/source-gcc/gcc/c-family/c-cppbuiltin.cc:1789: option `string_length' not supported for field `hex_str' of global `lazy_hex_fp_values[0]'

This amends commit f3b957ea8b9dadfb1ed30f24f463529684b7a36a
"pch: Fix streaming of strings with embedded null bytes".

	gcc/
	* gengtype.cc (write_root, write_roots): Explicitly reject
	'string_length' option.
	* doc/gty.texi (GTY Options) <string_length>: Document.