[AArch64] Add support for __jcvt intrinsic

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

	* 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.

	* gcc.target/aarch64/acle/jcvt_1.c: New test.

From-SVN: r275335