exec.c: Relax restrictions on watchpoint length and alignment

exec.c: Relax restrictions on watchpoint length and alignment

The current implementation of watchpoints requires that they
have a power of 2 length which is not greater than TARGET_PAGE_SIZE
and that their address is a multiple of their length. Watchpoints
on ARM don't fit these restrictions, so change the implementation
so they can be relaxed.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>