RISC-V: Add support for sdtrig and ssstrict extensions.

This implements the sdtrig extension, version 1.0[1] and ssstrict
extension, version 1.0[2].

[1]https://github.com/riscv/riscv-debug-spec/blob/main/Sdtrig.adoc
[2]https://github.com/riscv/riscv-profiles/issues/173

bfd/ChangeLog:

	* elfxx-riscv.c: Added sdtrig and ssstrict v1.0, and imply rules.

gas/ChangeLog:

	* NEWS: Updated for sdtrig and ssstrict.
	* testsuite/gas/riscv/imply.d: DItto.
	* testsuite/gas/riscv/imply.s: Ditto.
	* testsuite/gas/riscv/march-help.l: Ditto.