gdb: rename c_printchar as language_defn::printchar

This commit removes the global function c_printchar and moves the
implementation into language_defn::printchar.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* c-lang.c (c_printchar): Rename to...
	(language_defn::printchar): ...this.
	* c-lang.h (c_printchar): Delete declaration.
	* language.c (language_defn::printchar): Delete this
	implementation.  Is now implemented in c-lang.c.