[committed] Use single quote rather than backquote in RISC-V diagnostic

Similar to the other patch this morning, this fixes a warning that was causing
the RISC-V bootstrap to fail.

In this case the diagnostic used a backquote.  This changes it to a simple
single quote which the diagnostic framework won't complain about.

Committed to the trunk.

gcc/
	* common/config/riscv/riscv-common.cc (riscv_subset_list::add): Use
	single quote rather than backquote in diagnostic.