gdb/riscv: implement cannot_store_register gdbarch method

gdb/riscv: implement cannot_store_register gdbarch method

The x0 (zero) register is read-only on RISC-V.  Implement the
cannot_store_register gdbarch method to tell GDB this.

Without this method GDB will try to write to x0, and relies on the
target to ignore such writes.  If you are using a target that
complains (or throws an error) when writing to x0, this change will
prevent this from happening.

The gdb.arch/riscv-reg-aliases.exp test exercises writing to x0, and
will show the errors when using a suitable target.