Revert use of classify callback in i386 gdbarch_tdep.

Revert use of classify callback in i386 gdbarch_tdep.

This is no longer useful, as it was introduced to reuse the funcall
handling code in amd64-tdep.c in the context of x64-windows. But
we have since then changed the implementations to be completely
independent of each other.

This reverts the non-windows-specific part of the change called:
    amd64: Integer parameters in function calls on Windows
(the x64-windows portion has already been reverted)

gdb/ChangeLog:

	Revert:
	* i386-tdep.h (enum amd64_reg_class): New, moved here from
	amd64-tdep.c.
	(struct gdbarch_tdep): Add fields call_dummy_num_integer_regs,
	call_dummy_integer_regs, and classify.
	* amd64-tdep.h (amd64_classify): Add declaration.
	* amd64-tdep.c (amd64_dummy_call_integer_regs): New static constant.
	(amd64_reg_class): Delete, moved to i386-tdep.h.
	(amd64_classify): Make non-static.  Move declaration to amd64-tdep.h.
	Replace call to amd64_classify by call to tdep->classify.
	(amd64_push_arguments): Get the list of registers to use for
	passing integer parameters from the gdbarch tdep structure,
	rather than using a hardcoded one.  Replace calls to amd64_classify
	by calls to tdep->classify.
	(amd64_push_dummy_call): Get the register number used for
	the "hidden" argument from tdep->call_dummy_integer_regs.
	(amd64_init_abi): Initialize tdep->call_dummy_num_integer_regs
	and tdep->call_dummy_integer_regs.  Set tdep->classify.