gdb: Add riscv to list of architectures with a save_reggroup

gdb: Add riscv to list of architectures with a save_reggroup

The regcache cooked_read test needs to know which architectures have a
save_reggroup, riscv does and needs adding to the list.

gdb/ChangeLog:

	* regcache.c (cooked_read_test): Add riscv to the list of
	architectures that have a save_reggroup.