RISC-V: Add support for the Zvks ISA extension

Zvks is part of the vector crypto extensions.

Zvks is shorthand for the following set of extensions:
- Zvksed
- Zvksh
- Zvbb
- Zvkt

bfd/ChangeLog:

	* elfxx-riscv.c: Define Zvks extension.

gas/ChangeLog:

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

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