Don't die when gdb thinks XLEN is 64 but it's 32.

Don't die when gdb thinks XLEN is 64 but it's 32.

Instead, just give gdb what it asks for.
Also when gdb does a register write, let the user know that it's likely
misconfigured and tell them how to fix it.

This is probably as well as issue #72 can be fixed in spike.