gdb: change gdbarch_have_nonsteppable_watchpoint to bool

There is one unnecessary call to
set_gdbarch_have_nonsteppable_watchpoint that passes false (which is the
default), in arc-tdep.c, but I left it there because there is an
insightful comment and I didn't feel like deleting it.

Change-Id: Iccb9851c1cf28f48a075649b1dbc7c9388383df3
Approved-By: Tom Tromey <tom@tromey.com>