Add note about making sure you're running the gcc you built

[davidmalcolm]

gcc versus ./xgcc -B. when trying to debug a self-built gcc can be a source of confusion; we should spell this out as something to double-check.

[davidmalcolm]

Indeed, the examples in https://gcc-newbies-guide.readthedocs.io/en/latest/debugging.html use "gcc" i.e. are implicitly using the system gcc.