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.

gas/ChangeLog

@@ -1,3 +1,10 @@
+2019-12-12  H.J. Lu  <hongjiu.lu@intel.com>
+
+	* 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.
+
 2019-12-12  Alan Modra  <amodra@gmail.com>
 
 	* config/tc-aarch64.c (get_aarch64_insn): Avoid signed overflow.

gas/doc/internals.texi

@@ -1210,6 +1210,11 @@ If you do not define @code{md_relax_frag}, you may define
 machine independent code knows how to use such a table to relax PC relative
 references.  See @file{tc-m68k.c} for an example.  @xref{Relaxation}.
 
+@item md_generic_table_relax_frag
+@cindex md_generic_table_relax_frag
+If defined, it is a C statement that is invoked, instead of
+the default implementation, to scan @code{TC_GENERIC_RELAX_TABLE}.
+
 @item md_prepare_relax_scan
 @cindex md_prepare_relax_scan
 If defined, it is a C statement that is invoked prior to scanning

gas/write.c

@@ -2481,6 +2481,10 @@ write_object_file (void)
 }
 
 #ifdef TC_GENERIC_RELAX_TABLE
+#ifndef md_generic_table_relax_frag
+#define md_generic_table_relax_frag relax_frag
+#endif
+
 /* Relax a fragment by scanning TC_GENERIC_RELAX_TABLE.  */
 
 long
@@ -3031,7 +3035,8 @@ relax_segment (struct frag *segment_frag_root, segT segment, int pass)
 #ifdef TC_GENERIC_RELAX_TABLE
 		/* The default way to relax a frag is to look through
 		   TC_GENERIC_RELAX_TABLE.  */
-		growth = relax_frag (segment, fragP, stretch);
+		growth = md_generic_table_relax_frag (segment, fragP,
+						      stretch);
 #endif /* TC_GENERIC_RELAX_TABLE  */
 #endif
 		break;