infcall: refactor 'call_function_by_hand_dummy'

Extract out the code region that reserves stack space to a separate
function.

Fix the comment of 'call_function_by_hand_dummy' to remove reference
to the NARGS argument that was removed in commit (e71585ff "Use
gdb:array_view in call_function_by_hand & friends").

gdb/ChangeLog:
2019-10-23  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* infcall.c (call_function_by_hand_dummy): Fix the function
	comment.  And extract out a code section into...
	(reserve_stack_space): ...this new function.

Change-Id: I8938ef4134aff68a0a21724aaa2406bfe453438a