RISC-V: Add support for the Zvknc ISA extension

Zvknc is part of the vector crypto extensions.

Zvknc is shorthand for the following set of extensxions:
- Zvkn
- Zvbc

bfd/ChangeLog:

	* elfxx-riscv.c: Define Zvknc extension.

gas/ChangeLog:

	* testsuite/gas/riscv/zvknc.d: New test.
	* testsuite/gas/riscv/zvknc.s: New test.

Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>