gas: Add md_generic_table_relax_frag

gas: Add md_generic_table_relax_frag

Add md_generic_table_relax_frag for TC_GENERIC_RELAX_TABLE targets so
that a backend can extend relax_frag beyond TC_GENERIC_RELAX_TABLE.

	* write.c (md_generic_table_relax_frag): New.  Defined to
	relax_frag if not defined.
	(relax_segment): Call md_generic_table_relax_frag instead of
	relax_frag.