gdb: Add default frame methods to gdbarch

gdb: Add default frame methods to gdbarch

Supply default gdbarch methods for gdbarch_dummy_id,
gdbarch_unwind_pc, and gdbarch_unwind_sp.  This patch doesn't actually
convert any targets to use these methods, and so, there will be no
user visible changes after this commit.

The implementations for default_dummy_id and default_unwind_sp are
fairly straight forward, these just take on the pattern used by most
targets.  Once these default methods are in place then most targets
will be able to switch over.

The implementation for default_unwind_pc is also fairly straight
forward, but maybe needs some explanation.

This patch has gone through a number of iterations:

  https://sourceware.org/ml/gdb-patches/2018-03/msg00165.html
  https://sourceware.org/ml/gdb-patches/2018-03/msg00306.html
  https://sourceware.org/ml/gdb-patches/2018-06/msg00090.html
  https://sourceware.org/ml/gdb-patches/2018-09/msg00127.html

and the implementation of default_unwind_pc has changed over this
time.  Originally, I took an implementation like this:

    CORE_ADDR
    default_unwind_pc (struct gdbarch *gdbarch, struct frame_info *next_frame)
    {
      int pc_regnum = gdbarch_pc_regnum (gdbarch);
      return frame_unwind_register_unsigned (next_frame, pc_regnum);
    }

This is basically a clone of default_unwind_sp, but using $pc.  It was
pointed out that we could potentially do better, and in version 2 the
implementation became:

    CORE_ADDR
    default_unwind_pc (struct gdbarch *gdbarch, struct frame_info *next_frame)
    {
      struct type *type;
      int pc_regnum;
      CORE_ADDR addr;
      struct value *value;

      pc_regnum = gdbarch_pc_regnum (gdbarch);
      value = frame_unwind_register_value (next_frame, pc_regnum);
      type = builtin_type (gdbarch)->builtin_func_ptr;
      addr = extract_typed_address (value_contents_all (value), type);
      addr = gdbarch_addr_bits_remove (gdbarch, addr);
      release_value (value);
      value_free (value);
      return addr;
    }

The idea was to try split out some of the steps of unwinding the $pc,
steps that are on some (or many) targets no-ops, and so allow targets
that do override these methods, to make use of default_unwind_pc.

This implementation remained in place for version 2, 3, and 4.

However, I realised that I'd made a mistake, most targets simply use
frame_unwind_register_unsigned to unwind the $pc, and this throws an
error if the register value is optimized out or unavailable.  My new
proposed implementation doesn't do this, I was going to end up
breaking many targets.

I considered duplicating the code from frame_unwind_register_unsigned
that throws the errors into my new default_unwind_pc, however, this
felt really overly complex.  So, what I instead went with was to
simply revert back to using frame_unwind_register_unsigned.  Almost
all existing targets already use this. Some of the ones that don't can
be converted to, which means almost all targets could end up using the
default.

One addition I have made over the version 1 implementation is to add a
call to gdbarch_addr_bits_remove.  For most targets this is a no-op,
but for a handful, having this call in place will mean that they can
use the default method.  After all this, the new default_unwind_pc now
looks like this:

    CORE_ADDR
    default_unwind_pc (struct gdbarch *gdbarch, struct frame_info *next_frame)
    {
      int pc_regnum = gdbarch_pc_regnum (gdbarch);
      CORE_ADDR pc = frame_unwind_register_unsigned (next_frame, pc_regnum);
      pc = gdbarch_addr_bits_remove (gdbarch, pc);
      return pc;
    }

gdb/ChangeLog:

	* gdb/dummy-frame.c (default_dummy_id): Defined new function.
	* gdb/dummy-frame.h (default_dummy_id): Declare new function.
	* gdb/frame-unwind.c (default_unwind_pc): Define new function.
	(default_unwind_sp): Define new function.
	* gdb/frame-unwind.h (default_unwind_pc): Declare new function.
	(default_unwind_sp): Declare new function.
	* gdb/frame.c (frame_unwind_pc): Assume gdbarch_unwind_pc is
	available.
	(get_frame_sp): Assume that gdbarch_unwind_sp is available.
	* gdb/gdbarch.c: Regenerate.
	* gdb/gdbarch.h: Regenerate.
	* gdb/gdbarch.sh: Update definition of dummy_id, unwind_pc, and
	unwind_sp.  Add additional header files to be included in
	generated file.