If an int 1587 call is made from an option rom, stay in bigreal mode.

Modify the int 1587 handler to check if the POST phase is still
running.  If it is, use bigreal mode segment limits so that the caller
remains in bigreal mode when the 1587 handler completes.  This helps
with SeaVGABIOS' use of "direct" framebuffer accesses (an option rom
may attempt to display text during its option rom execution which can
cause SeaVGABIOS to make the int 1587 calls).

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>