RISC-V: Add support for ssdbltrp and smdbltrp extension.

This implements the ssdbltrp extensons, version 1.0[1] and the smdbltrp
extensions, version1.0[2].

[1] https://github.com/riscv/riscv-isa-manual/blob/main/src/ssdbltrp.adoc
[2] https://github.com/riscv/riscv-isa-manual/blob/main/src/smdbltrp.adoc

bfd/ChangeLog:

	* elfxx-riscv.c: Add 'ssdbltrp' and 'smdbltrp' to the list of konwn
	  standard extensions.

gas/ChangeLog:

	* NEWS: Updated.
	* testsuite/gas/riscv/imply.d: Ditto.
	* testsuite/gas/riscv/imply.s: Ditto.
	* testsuite/gas/riscv/march-help.l: Ditto.