aarch64: Relax check for RNG system registers

FEAT_RNG is an optional Armv8.5-A extension, but it can be backported
to earlier architectures as well.  GAS previously made the RNG registers
conditional on having both armv8.5-a and +rng, but only +rng should be
required.

This seems to be the only feature that was handled like this.

opcodes/
	* aarch64-opc.c (SR_RNG): Don't require V8_5.

gas/
	* testsuite/gas/aarch64/rng-1.s, testsuite/gas/aarch64/rng-1.d: New
	test.