[PATCH][AArch64][GCC 9] Add support for __jcvt intrinsic

[Kyrylo Tkachov]

Hi all,

I'd like to backport support for the __jcvt intrinsic to the active branches as it's an Armv8.3-a intrinsic that should have been supported there.
This is a squashed commit of the initial supported and a couple of follow-up fixes from Andrea.
This is the GCC 9 version.

Bootstrapped and tested on the branch.

This patch implements the __jcvt ACLE intrinsic [1] that maps down to the FJCVTZS [2] instruction from Armv8.3-a.
No fancy mode iterators or nothing. Just a single builtin, UNSPEC and define_insn and the associate plumbing.
This patch also defines __ARM_FEATURE_JCVT to indicate when the intrinsic is available.

[1] https://developer.arm.com/docs/101028/latest/data-processing-intrinsics
[2] https://developer.arm.com/docs/ddi0596/latest/simd-and-floating-point-instructions-alphabetic-order/fjcvtzs-floating-point-javascript-convert-to-signed-fixed-point-rounding-toward-zero

gcc/
	PR target/71233
	* config/aarch64/aarch64.md (UNSPEC_FJCVTZS): Define.
	(aarch64_fjcvtzs): New define_insn.
	* config/aarch64/aarch64.h (TARGET_JSCVT): Define.
	* config/aarch64/aarch64-builtins.c (aarch64_builtins):
	Add AARCH64_JSCVT.
	(aarch64_init_builtins): Initialize __builtin_aarch64_jcvtzs.
	(aarch64_expand_builtin): Handle AARCH64_JSCVT.
	* config/aarch64/aarch64-c.c (aarch64_update_cpp_builtins): Define
	__ARM_FEATURE_JCVT where appropriate.
	* config/aarch64/arm_acle.h (__jcvt): Define.
	* doc/sourcebuild.texi (aarch64_fjcvtzs_hw) Document new
	target supports option.

gcc/testsuite/
	PR target/71233
	* gcc.target/aarch64/acle/jcvt_1.c: New test.
	* gcc.target/aarch64/acle/jcvt_2.c: New testcase.
	* lib/target-supports.exp
	(check_effective_target_aarch64_fjcvtzs_hw): Add new check for
	FJCVTZS hw.

Co-Authored-By: Andrea Corallo  <andrea.corallo@arm.com>

(cherry picked from commit e1d5d19ec4f84b67ac693fef5b2add7dc9cf056d)
(cherry picked from commit 2c62952f8160bdc8d4111edb34a4bc75096c1e05)
(cherry picked from commit d2b86e14c14020f3e119ab8f462e2a91bd7d46e5)
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