RISC-V: Support Svrsw60t59b extension.

This patch adds RISC-V Svrsw60t59b extension minimal support.

https://github.com/riscv/riscv-isa-manual/blob/f30292bb486d735e186f06f705fcd7beff1e3e97/src/supervisor.adoc#L2550

bfd/ChangeLog:

	* elfxx-riscv.c: New ext.

gas/ChangeLog:

	* NEWS: New ext.
	* testsuite/gas/riscv/march-help.l: Ditto.