Add a new target hook: TARGET_START_CALL_ARGS

We have the following two hooks into the call expansion code:

- TARGET_CALL_ARGS is called for each argument before arguments
  are moved into hard registers.

- TARGET_END_CALL_ARGS is called after the end of the call
  sequence (specifically, after any return value has been
  moved to a pseudo).

This patch adds a TARGET_START_CALL_ARGS hook that is called before
the TARGET_CALL_ARGS sequence.  This means that TARGET_START_CALL_REGS
and TARGET_END_CALL_REGS bracket the region in which argument registers
might be live.  They also bracket a region in which the only call
emiitted by target-independent code is the call to the target function
itself.  (For example, TARGET_START_CALL_ARGS happens after any use of
memcpy to copy arguments, and TARGET_END_CALL_ARGS happens before any
use of memcpy to copy the result.)

Also, the patch adds the cumulative argument structure as an argument
to the hooks, so that the target can use it to record and retrieve
information about the call as a whole.

The TARGET_CALL_ARGS docs said:

   While generating RTL for a function call, this target hook is invoked once
   for each argument passed to the function, either a register returned by
   ``TARGET_FUNCTION_ARG`` or a memory location.  It is called just
-  before the point where argument registers are stored.

The last bit was true for normal calls, but for libcalls the hook was
invoked earlier, before stack arguments have been copied.  I don't think
this caused a practical difference for nvptx (the only port to use the
hooks) since I wouldn't expect any libcalls to take stack parameters.

gcc/
	* doc/tm.texi.in: Add TARGET_START_CALL_ARGS.
	* doc/tm.texi: Regenerate.
	* target.def (start_call_args): New hook.
	(call_args, end_call_args): Add a parameter for the cumulative
	argument information.
	* hooks.h (hook_void_rtx_tree): Delete.
	* hooks.cc (hook_void_rtx_tree): Likewise.
	* targhooks.h (hook_void_CUMULATIVE_ARGS): Declare.
	(hook_void_CUMULATIVE_ARGS_rtx_tree): Likewise.
	* targhooks.cc (hook_void_CUMULATIVE_ARGS): New function.
	(hook_void_CUMULATIVE_ARGS_rtx_tree): Likewise.
	* calls.cc (expand_call): Call start_call_args before computing
	and storing stack parameters.  Pass the cumulative argument
	information to call_args and end_call_args.
	(emit_library_call_value_1): Likewise.
	* config/nvptx/nvptx.cc (nvptx_call_args): Add a cumulative
	argument parameter.
	(nvptx_end_call_args): Likewise.

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