gdb: Prepare watchpoints_t data structure

gdb: Prepare watchpoints_t data structure

In preparation for the watchpoint implementation define an
avr_gdb_watchpoints_t struct to handle both. The struct is only accessed
through gdb_watch_* functions, therefore changing its internals should
be easy.

Note that this implementation cannot handle different watchpoints at the
same address with a different size.

The avr_gdb_watch_type enum is public and will be used in sim_core.c in
a follow-up commit.