Update TARGET_FUNCTION_INCOMING_ARG documentation

On x86, interrupt handlers are only called by processors which push
interrupt data onto stack at the address where the normal return address
is.  Since interrupt handlers must access interrupt data via pointers so
that they can update interrupt data, the pointer argument is passed as
"argument pointer - word".

TARGET_FUNCTION_INCOMING_ARG defines how callee sees its argument.
Normally it returns REG, NULL, or CONST_INT.  This patch adds arbitrary
address computation based on hard register, which can be forced into a
register, to the list.

When copying an incoming argument onto stack, assign_parm_setup_stack
has:

if (argument in memory)
  copy argument in memory to stack
else
  move argument to stack

Since an arbitrary address computation may be passed as an argument, we
change it to:

if (argument in memory)
  copy argument in memory to stack
else
  {
    if (argument isn't in register)
      force argument into a register
    move argument to stack
  }

	* function.c (assign_parm_setup_stack): Force source into a
	register if needed.
	* target.def (function_incoming_arg): Update documentation to
	allow arbitrary address computation based on hard register.
	* doc/tm.texi: Regenerated.

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