compat32: Implement PT_GETDBREGS and PT_SETDBREGS

Uncomment and improve the implementation of compat32 support for
PT_GETDBREGS and PT_SETDBREGS requests.

The new implementation uses x86_dbregs_read() and x86_dbregs_write()
function instead of accessing pcb directly.  While this might be
a little slower, it guarantees that the needed pcb field is allocated
correctly.

Furthermore, the code introduces necessary sanity checks
for PT_SETDBREGS arguments.

(mgorny)