RISC-V: Add support for the Zvkn ISA extension

Zvkn is part of the vector crypto extensions.

Zvkn is shorthand for the following set of extensions:
- Zvkned
- Zvknhb
- Zvbb
- Zvkt

bfd/ChangeLog:

	* elfxx-riscv.c: Define Zvkn extension.

gas/ChangeLog:

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

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