aarch64: Add WFET instruction for Armv8.7-a

This patch adds new to Armv8.7 WFET instruction which take one operand:

    WFET <Xt>

Where:
    <Xt> is 64-bit name of the general-purpose source register, encoded in the
         "Rd" field.

For more details regarding WFET (Wait For Event with Timeout) instruction for
Armv8.7-a please refer to Arm A64 Instruction set documentation for Armv8-A
architecture profile, see document pages 565 of [0].

	[0]: https://developer.arm.com/docs/ddi0596/i

gas/ChangeLog:

2020-10-27  Przemyslaw Wirkus  <przemyslaw.wirkus@arm.com>

	* NEWS: Update docs.
	* testsuite/gas/aarch64/system-5.d: New test.
	* testsuite/gas/aarch64/system-5.s: New test.

opcodes/ChangeLog:

2020-10-27  Przemyslaw Wirkus  <przemyslaw.wirkus@arm.com>

	* aarch64-tbl.h (struct aarch64_opcode): Add new WFET instruction encoding
	and operand description.
	* aarch64-asm-2.c: Regenerated.
	* aarch64-dis-2.c: Regenerated.
	* aarch64-opc-2.c: Regenerated.