[AArch64] Implement __rndr, __rndrrs intrinsics

[AArch64] Implement __rndr, __rndrrs intrinsics

This patch implements the recently published[1] __rndr and __rndrrs
intrinsics used to access the RNG in Armv8.5-A.
The __rndrrs intrinsics can be used to reseed the generator too.
They are guarded by the __ARM_FEATURE_RNG feature macro.
A quirk with these intrinsics is that they store the random number in
their pointer argument and return a status
code if the generation succeeded.

The instructions themselves write the CC flags indicating the success of
the operation that we can then read with a CSET.
Therefore this implementation makes use of the IGNORE indicator to the
builtin expand machinery to avoid generating
the CSET if its result is unused (the CC reg clobbering effect is still
reflected in the pattern).
I've checked that using unspec_volatile prevents undesirable CSEing of
the instructions.

[1] https://developer.arm.com/docs/101028/latest/data-processing-intrinsics

	* config/aarch64/aarch64.md (UNSPEC_RNDR, UNSPEC_RNDRRS): Define.
	(aarch64_rndr): New define_insn.
	(aarch64_rndrrs): Likewise.
	* config/aarch64/aarch64.h (AARCH64_ISA_RNG): Define.
	(TARGET_RNG): Likewise.
	* config/aarch64/aarch64.c (aarch64_expand_builtin): Use IGNORE
	argument.
	* config/aarch64/aarch64-protos.h (aarch64_general_expand_builtin):
	Add fourth argument in prototype.
	* config/aarch64/aarch64-builtins.c (enum aarch64_builtins):
	Add AARCH64_BUILTIN_RNG_RNDR, AARCH64_BUILTIN_RNG_RNDRRS.
	(aarch64_init_rng_builtins): Define.
	(aarch64_general_init_builtins): Call aarch64_init_rng_builtins.
	(aarch64_expand_rng_builtin): Define.
	(aarch64_general_expand_builtin): Use IGNORE argument, handle
	RNG builtins.
	* config/aarch64/aarch64-c.c (aarch64_update_cpp_builtins): Define
	__ARM_FEATURE_RNG when TARGET_RNG.
	* config/aarch64/arm_acle.h (__rndr, __rndrrs): Define.

	* gcc.target/aarch64/acle/rng_1.c: New test.

From-SVN: r277239