base: Make the default checkBpLen method return true.

base: Make the default checkBpLen method return true.

This was checking that the breakpoint length was equal to the length of
the ISA specific MachInst type. Instead, force the ISA specific remote
GDB subclass to implement a check if it wants to, specific to its needs.
The base implementation will just approve of any length, which should be
fine with a well behaved GDB client.

Change-Id: Id7325b788f8445049855f8104082b8e4da1fe300
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39661
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>