gdb/python: handle saving user registers in a frame unwinder

This patch came about because I wanted to write a frame unwinder that
would corrupt the backtrace in a particular way.  In order to achieve
what I wanted I ended up trying to write an unwinder like this:

  class FrameId(object):
      .... snip class definition ....

  class TestUnwinder(Unwinder):
      def __init__(self):
          Unwinder.__init__(self, "some name")

      def __call__(self, pending_frame):
          pc_desc = pending_frame.architecture().registers().find("pc")
          pc = pending_frame.read_register(pc_desc)

          sp_desc = pending_frame.architecture().registers().find("sp")
          sp = pending_frame.read_register(sp_desc)

          # ... snip code to decide if this unwinder applies or not.

          fid = FrameId(pc, sp)
          unwinder = pending_frame.create_unwind_info(fid)
          unwinder.add_saved_register(pc_desc, pc)
          unwinder.add_saved_register(sp_desc, sp)
          return unwinder

The important things here are the two calls:

          unwinder.add_saved_register(pc_desc, pc)
          unwinder.add_saved_register(sp_desc, sp)

On x86-64 these would fail with an assertion error:

  gdb/regcache.c:168: internal-error: int register_size(gdbarch*, int): Assertion `regnum >= 0 && regnum < gdbarch_num_cooked_regs (gdbarch)' failed.

What happens is that in unwind_infopy_add_saved_register (py-unwind.c)
we call register_size, as register_size should only be called on
cooked (real or pseudo) registers, and 'pc' and 'sp' are implemented
as user registers (at least on x86-64), we trigger the assertion.

A simple fix would be to check in unwind_infopy_add_saved_register if
the register number we are handling is a cooked register or not, if
not we can throw a 'Bad register' error back to the Python code.

However, I think we can do better.

Consider that at the CLI we can do this:

  (gdb) set $pc=0x1234

This works because GDB first evaluates '$pc' to get a register value,
then evaluates '0x1234' to create a value encapsulating the
immediate.  The contents of the immediate value are then copied back
to the location of the register value representing '$pc'.

The value location for a user-register will (usually) be the location
of the real register that was accessed, so on x86-64 we'd expect this
to be $rip.

So, in this patch I propose that in the unwinder code, when
add_saved_register is called, if it is passed a
user-register (i.e. non-cooked) then we first fetch the register,
extract the real register number from the value's location, and use
that new register number when handling the add_saved_register call.

If either the value location that we get for the user-register is not
a cooked register then we can throw a 'Bad register' error back to the
Python code, but in most cases this will not happen.

gdb/ChangeLog:

	* python/py-unwind.c (unwind_infopy_add_saved_register): Handle
	saving user registers.

gdb/testsuite/ChangeLog:

	* gdb.python/py-unwind-user-regs.c: New file.
	* gdb.python/py-unwind-user-regs.exp: New file.
	* gdb.python/py-unwind-user-regs.py: New file.

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