gdbsupport/tdesc: print enum fields using 'evalue' syntax

Currently when printing an XML description GDB prints enum values like
this:

  <enum id="levels_type" size="4">
    <field name="low" start="0"/>
    <field name="high" start="1"/>
  </enum>

This is incorrect, and is most likely a copy and paste error with the
struct and flags printing code.  The correct syntax is:

  <enum id="levels_type" size="4">
    <evalue name="low" value="0"/>
    <evalue name="high" value="1"/>
  </enum>

A test is included to cover this functionality.

gdb/testsuite/ChangeLog:

	* gdb.xml/maint-xml-dump-03.xml: New file.

gdbsupport/ChangeLog:

	* tdesc.cc (print_xml_feature::visit): Print enum fields using
	'evalue' syntax.

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2020-12-01  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* gdb.xml/maint-xml-dump-03.xml: New file.
+
 2020-11-30  Tom de Vries  <tdevries@suse.de>
 
 	PR symtab/26905

gdb/testsuite/gdb.xml/maint-xml-dump-03.xml

@@ -0,0 +1,13 @@
+<target>
+  <feature name="abc">
+    <enum id="levels_type" size="4">
+      <evalue name="low" value="0"/>
+      <evalue name="high" value="1"/>
+    </enum>
+    <flags id="flags_type" size="4">
+      <field name="X" start="0" end="0" type="bool"/>
+      <field name="LEVEL" start="1" end="1" type="levels_type"/>
+    </flags>
+    <reg name="flags" bitsize="32" type="flags_type" regnum="0"/>
+  </feature>
+</target>

gdbsupport/ChangeLog

@@ -1,3 +1,8 @@
+2020-12-01  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* tdesc.cc (print_xml_feature::visit): Print enum fields using
+	'evalue' syntax.
+
 2020-12-01  Chungyi Chi  <demonic@csie.io>
 
 	* tdesc.cc (print_xml_feature::visit): Print enum size attribute.

gdbsupport/tdesc.cc

@@ -355,8 +355,10 @@ void print_xml_feature::visit (const tdesc_type_with_fields *t)
 	string_appendf (tmp, " size=\"%d\"", t->size);
       string_appendf (tmp, ">");
       add_line (tmp);
+      /* The 'start' of the field is reused as the enum value.  The 'end'
+	 of the field is always set to -1 for enum values.  */
       for (const tdesc_type_field &f : t->fields)
-	add_line ("  <field name=\"%s\" start=\"%d\"/>",
+	add_line ("  <evalue name=\"%s\" value=\"%d\"/>",
 		  f.name.c_str (), f.start);
       break;