exec: flush the whole TLB if a watchpoint crosses a page boundary

exec: flush the whole TLB if a watchpoint crosses a page boundary

There is no particular reason why you can't have a watchpoint in TCG
that covers a large chunk of the address space. We could be clever
about it but these cases are pretty rare and we can assume the user
will expect a little performance degradation.

NB: In my testing gdb will silently squash a watchpoint like:

  watch (char[0x7fffffffff]) *0x0

to a 4 byte watchpoint. Practically it will limit the maximum size
based on max-value-size. However given enough of a tweak the sky is
the limit.

Reported-by: Alexander Bulekov <[email protected]>
Signed-off-by: Alex Bennée <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-Id: <20200605154929[email protected]>