RISC-V: Add Smrnmi extension imply relation.

This patch adds the dependency of Smrnmi extension on Zicsr extension.

bfd/ChangeLog:

	* elfxx-riscv.c: New imply.

gas/ChangeLog:

	* testsuite/gas/riscv/imply.d: New test check.
	* testsuite/gas/riscv/imply.s: New imply test.

Signed-off-by: Jiawei <jiawei@iscas.ac.cn>