RISC-V: Add support for the Zvksc ISA extension

Zvksc is part of the vector crypto extensions.

Zvksc is shorthand for the following set of extensions:
- Zvks
- Zvbc

bfd/ChangeLog:

	* elfxx-riscv.c: Define Zvksc extension.

gas/ChangeLog:

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

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