accel/tcg: Handle get_page_addr_code() returning -1 in tb_check_watchpoint()

accel/tcg: Handle get_page_addr_code() returning -1 in tb_check_watchpoint()

When we support execution from non-RAM MMIO regions, get_page_addr_code()
will return -1 to indicate that there is no RAM at the requested address.
Handle this in tb_check_watchpoint() -- if the exception happened for a
PC which doesn't correspond to RAM then there is no need to invalidate
any TBs, because the one-instruction TB will not have been cached.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Tested-by: Cédric Le Goater <[email protected]>
Message-id: 20180710160013[email protected]