[ARM] Enable descriptors for nested functions in Ada

[Eric Botcazou]

Similarly to x86, PowerPC and SPARC, this enables the use of custom run-time 
descriptors in Ada, thus eliminating the need for trampolines and executable 
stack in presence of pointers to nested functions.

This still uses bit 1 for the run-time identification scheme because bumping 
the function alignment to 64 bits seems undesirable in Thumb mode.

Tested on ARM/Linux, OK for the mainline?


2016-11-13  Eric Botcazou  <ebotcazou@adacore.com>

	PR ada/67205
	* config/arm/arm.c (TARGET_CUSTOM_FUNCTION_DESCRIPTORS): Define.
	(arm_function_ok_for_sibcall): Return false for an indirect call by
	descriptor if all the argument registers are used.
	(arm_relayout_function): Use FUNCTION_ALIGNMENT macro to adjust the
	alignment of the function.

-- 
Eric Botcazou
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