Add a target hook for getting an ABI from a function type

Add a target hook for getting an ABI from a function type

This patch adds a target hook that allows targets to return
the ABI associated with a particular function type.  Generally,
when multiple ABIs are in use, it must be possible to tell from
a function type and its attributes which ABI it is using.

2019-09-30  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* target.def (fntype_abi): New target hook.
	* doc/tm.texi.in (TARGET_FNTYPE_ABI): Likewise.
	* doc/tm.texi: Regenerate.
	* target.h (predefined_function_abi): Declare.
	* function-abi.cc (fntype_abi): Call targetm.calls.fntype_abi,
	if defined.
	* config/aarch64/aarch64.h (ARM_PCS_SIMD): New arm_pcs value.
	* config/aarch64/aarch64.c: Include function-abi.h.
	(aarch64_simd_abi, aarch64_fntype_abi): New functions.
	(TARGET_FNTYPE_ABI): Define.

From-SVN: r276308