ld: new options --ctf-variables and --ctf-share-types

ld: new options --ctf-variables and --ctf-share-types

libctf recently changed to make it possible to not emit the CTF
variables section.  Make this the default for ld: the variables section
is a simple name -> type mapping, and the names can be quite voluminous.
Nothing in the variables section appears in the symbol table, by
definition, so GDB cannot make use of them: special-purpose projects
that implement their own analogues of symbol table lookup can do so, but
they'll need to tell the linker to emit the variables section after all.

The new --ctf-variables option does this.

The --ctf-share-types option (valid values "share-duplicated" and
"share-unconflicted") allow the caller to specify the CTF link mode.
Most users will want share-duplicated, since it allows for more
convenient debugging: but very large projects composed of many decoupled
components may want to use share-unconflicted mode, which places types
that appear in only one TU into per-TU dicts.  (They may also want to
relink the CTF using the ctf_link API and cu-mapping, to make their
"components" larger than a single TU.  Right now the linker does not
expose the CU-mapping machinery.  Perhaps it should in future to make
this use case easier.)

For now, giving the linker the ability to emit share-duplicated CTF lets
us add testcases for that mode to the testsuite.

ld/
	* ldlex.h (option_values) <OPTION_CTF_VARIABLES,
	OPTION_NO_CTF_VARIABLES, OPTION_CTF_SHARE_TYPES>: New.
	* ld.h (ld_config_type) <ctf_variables, ctf_share_duplicated>:
	New fields.
	* ldlang.c (lang_merge_ctf): Use them.
	* lexsup.c (ld_options): Add ctf-variables, no-ctf-variables,
	ctf-share-types.
	(parse_args) <OPTION_CTF_VARIABLES, OPTION_NO_CTF_VARIABLES,
	OPTION_CTF_SHARE_TYPES>: New cases.
	* ld.texi: Document new options.
	* NEWS: Likewise.