gdb: refactor the non-printing disassemblers

This commit started from an observation I made while working on some
other disassembler patches, that is, that the function
gdb_buffered_insn_length, is broken ... sort of.

I noticed that the gdb_buffered_insn_length function doesn't set up
the application data field if the disassemble_info structure.

Further, I noticed that some architectures, for example, ARM, require
that the application_data field be set, see gdb_print_insn_arm in
arm-tdep.c.

And so, if we ever use gdb_buffered_insn_length for ARM, then GDB will
likely crash.  Which is why I said only "sort of" broken.  Right now
we don't use gdb_buffered_insn_length with ARM, so maybe it isn't
broken yet?

Anyway to prove to myself that there was a problem here I extended the
disassembler self tests in disasm-selftests.c to include a test of
gdb_buffered_insn_length.  As I run the test for all architectures, I
do indeed see GDB crash for ARM.

To fix this we need gdb_buffered_insn_length to create a disassembler
that inherits from gdb_disassemble_info, but we also need this new
disassembler to not print anything.

And so, I introduce a new gdb_non_printing_disassembler class, this is
a disassembler that doesn't print anything to the output stream.

I then observed that both ARC and S12Z also create non-printing
disassemblers, but these are slightly different.  While the
disassembler in gdb_non_printing_disassembler reads the instruction
from a buffer, the ARC and S12Z disassemblers read from target memory
using target_read_code.

And so, I further split gdb_non_printing_disassembler into two
sub-classes, gdb_non_printing_memory_disassembler and
gdb_non_printing_buffer_disassembler.

The new selftests now pass, but otherwise, there should be no user
visible changes after this commit.

gdb/disasm.h

@@ -136,6 +136,56 @@ protected:
     ATTRIBUTE_PRINTF(3,4);
 };
 
+/* A basic disassembler that doesn't actually print anything.  */
+
+struct gdb_non_printing_disassembler : public gdb_disassemble_info
+{
+  gdb_non_printing_disassembler (struct gdbarch *gdbarch,
+				 read_memory_ftype read_memory_func)
+    : gdb_disassemble_info (gdbarch, nullptr /* stream */,
+			    read_memory_func,
+			    nullptr /* memory_error_func */,
+			    nullptr /* print_address_func */,
+			    null_fprintf_func,
+			    null_fprintf_styled_func)
+  { /* Nothing.  */ }
+
+private:
+
+  /* Callback used as the disassemble_info's fprintf_func callback, this
+     doesn't write anything to STREAM, but just returns 0.  */
+  static int null_fprintf_func (void *stream, const char *format, ...)
+    ATTRIBUTE_PRINTF(2,3);
+
+  /* Callback used as the disassemble_info's fprintf_styled_func callback,
+     , this doesn't write anything to STREAM, but just returns 0.  */
+  static int null_fprintf_styled_func (void *stream,
+				       enum disassembler_style style,
+				       const char *format, ...)
+    ATTRIBUTE_PRINTF(3,4);
+};
+
+/* 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
+{
+  /* 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 dissassembler class that provides 'print_insn', a method for
    disassembling a single instruction to the output stream.  */
 
@@ -278,10 +328,4 @@ extern char *get_disassembler_options (struct gdbarch *gdbarch);
 
 extern void set_disassembler_options (const char *options);
 
-/* Setup DINFO with its output function and output stream setup so that
-   nothing is printed while disassembling.  */
-
-extern void init_disassemble_info_for_no_printing
-  (struct disassemble_info *dinfo);
-
 #endif