* c-common.c (handle_visibility_attribute): Accept "default".
	* tree.h (enum symbol_visibility): New.
	(decl_visibility): Declare.
	* target.h (gcc_target.visibility): Take visibility arg as integer.
	* varasm.c (default_assemble_visibility): Likewise.
	(decl_visibility): New.
	(maybe_assemble_visibility): Use it.
	* output.h (default_assemble_visibility): Update prototype.
	* config/rs6000/rs6000.c (rs6000_assemble_visibility): Take
	visibility arg as integer.
	* doc/extend.texi: Document default visibility.

From-SVN: r59560