gdb: unify two dis_asm_read_memory functions in disasm.c

After the recent restructuring of the disassembler code, GDB has ended
up with two identical class static functions, both called
dis_asm_read_memory, with identical implementations.

My first thought was to move these out of their respective classes,
and just make them global functions, then I'd only need a single
copy.

And maybe that's the right way to go.  But I disliked that by doing
that I loose the encapsulation of the method with the corresponding
disassembler class.

So, instead, I placed the static method into its own class, and had
both the gdb_non_printing_memory_disassembler and gdb_disassembler
classes inherit from this new class as an additional base-class.

In terms of code generated, I don't think there's any significant
difference with this approach, but I think this better reflects how
the function is closely tied to the disassembler.

There should be no user visible changes after this commit.

gdb/disasm.c

@@ -132,8 +132,8 @@ line_has_code_p (htab_t table, struct symtab *symtab, int line)
 /* Wrapper of target_read_code.  */
 
 int
-gdb_disassembler::dis_asm_read_memory (bfd_vma memaddr, gdb_byte *myaddr,
+gdb_disassembler_memory_reader::dis_asm_read_memory
-				       unsigned int len,
+  (bfd_vma memaddr, gdb_byte *myaddr, unsigned int len,
    struct disassemble_info *info)
 {
   return target_read_code (memaddr, myaddr, len);
@@ -1021,16 +1021,6 @@ gdb_non_printing_disassembler::null_fprintf_styled_func
   return 0;
 }
 
-/* See disasm.h.  */
-
-int
-gdb_non_printing_memory_disassembler::dis_asm_read_memory
-  (bfd_vma memaddr, bfd_byte *myaddr, unsigned int length,
-   struct disassemble_info *dinfo)
-{
-  return target_read_code (memaddr, myaddr, length);
-}
-
 /* A non-printing disassemble_info management class.  The disassemble_info
    setup by this class will not print anything to the output stream (there
    is no output stream), and the instruction to be disassembled will be

gdb/disasm.h

@@ -165,31 +165,39 @@ private:
     ATTRIBUTE_PRINTF(3,4);
 };
 
-/* A non-printing disassemble_info management class.  The disassemble_info
+/* This is a helper class, for use as an additional base-class, by some of
-   setup by this class will not print anything to the output stream (there
+   the disassembler classes below.  This class just defines a static method
-   is no output stream), and the instruction to be disassembled will be
+   for reading from target memory, which can then be used by the various
-   read from target memory.  */
+   disassembler sub-classes.  */
 
-struct gdb_non_printing_memory_disassembler
+struct gdb_disassembler_memory_reader
-  : public gdb_non_printing_disassembler
 {
-  /* Constructor.  GDBARCH is the architecture to disassemble for.  */
-  gdb_non_printing_memory_disassembler (struct gdbarch *gdbarch)
-    :gdb_non_printing_disassembler (gdbarch, dis_asm_read_memory)
-  { /* Nothing.  */ }
-
-private:
-
   /* Implements the read_memory_func disassemble_info callback.  */
   static int dis_asm_read_memory (bfd_vma memaddr, gdb_byte *myaddr,
 				  unsigned int len,
 				  struct disassemble_info *info);
 };
 
+/* A non-printing disassemble_info management class.  The disassemble_info
+   setup by this class will not print anything to the output stream (there
+   is no output stream), and the instruction to be disassembled will be
+   read from target memory.  */
+
+struct gdb_non_printing_memory_disassembler
+  : public gdb_non_printing_disassembler,
+    private gdb_disassembler_memory_reader
+{
+  /* Constructor.  GDBARCH is the architecture to disassemble for.  */
+  gdb_non_printing_memory_disassembler (struct gdbarch *gdbarch)
+    :gdb_non_printing_disassembler (gdbarch, dis_asm_read_memory)
+  { /* Nothing.  */ }
+};
+
 /* A dissassembler class that provides 'print_insn', a method for
    disassembling a single instruction to the output stream.  */
 
-struct gdb_disassembler : public gdb_printing_disassembler
+struct gdb_disassembler : public gdb_printing_disassembler,
+			  private gdb_disassembler_memory_reader
 {
   gdb_disassembler (struct gdbarch *gdbarch, struct ui_file *file)
     : gdb_disassembler (gdbarch, file, dis_asm_read_memory)
@@ -239,9 +247,6 @@ private:
      (currently just to addresses and symbols) as it goes.  */
   static bool use_ext_lang_colorization_p;
 
-  static int dis_asm_read_memory (bfd_vma memaddr, gdb_byte *myaddr,
-				  unsigned int len,
-				  struct disassemble_info *info);
   static void dis_asm_memory_error (int err, bfd_vma memaddr,
 				    struct disassemble_info *info);
   static void dis_asm_print_address (bfd_vma addr,