gdb: Convert language la_printstr field to a method

This commit changes the language_data::la_printstr function pointer
member variable into a member function of language_defn.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* ada-lang.c (ada_language_data): Delete la_printstr initializer.
	(ada_language::printstr): New member function.
	* c-lang.c (c_language_data): Delete la_printstr initializer.
	(cplus_language_data): Likewise.
	(asm_language_data): Likewise.
	(minimal_language_data): Likewise.
	* d-lang.c (d_language_data): Likewise.
	* f-lang.c (f_printstr): Rename to f_language::printstr.
	(f_language_data): Delete la_printstr initializer.
	(f_language::printstr): New member function, implementation from
	f_printstr.
	* go-lang.c (go_language_data): Delete la_printstr initializer.
	* language.c (language_defn::printstr): Define new member
	function.
	(unk_lang_printstr): Delete.
	(unknown_language_data): Delete la_printstr initializer.
	(unknown_language::printstr): New member function.
	(auto_language_data): Delete la_printstr initializer.
	(auto_language::printstr): New member function.
	* language.h (language_data): Delete la_printstr field.
	(language_defn::printstr): Declare new member function.
	(LA_PRINT_STRING): Update call to printstr.
	* m2-lang.c (m2_printstr): Rename to m2_language::printstr.
	(m2_language_data): Delete la_printstr initializer.
	(m2_language::printstr): New member function, implementation from
	m2_printstr.
	* objc-lang.c (objc_language_data): Delete la_printstr
	initializer.
	* opencl-lang.c (opencl_language_data): Likewise.
	* p-lang.c (pascal_printstr): Rename to pascal_language::printstr.
	(pascal_language_data): Delete la_printstr initializer.
	(pascal_language::printstr): New member function, implementation
	from pascal_printstr.
	* p-lang.h (pascal_printstr): Delete declaration.
	* rust-lang.c (rust_printstr): Update header comment.
	(rust_language_data): Delete la_printstr initializer.
	(rust_language::printstr): New member function.