[librm] Allow for the PIC interrupt vector offset to be changed

Some external code (observed with FreeBSD's bootloader) will continue
to make INT 13 calls after reconfiguring the 8259 PIC to change the
vector offsets for IRQs.  If an IRQ (e.g. the timer IRQ) subsequently
occurs while iPXE is in protected mode, this will cause a general
protection fault since the corresponding IDT entry is empty.

A general protection fault is INT 0x0d, which happens to overlap with
the original IRQ5.  We therefore do have an ISR set up to handle a
general protection fault, but this ISR simply reflects the interrupt
down to the real-mode INT 0x0d and then attempts to return.  Since our
ISR is expecting a hardware interrupt rather than a general protection
fault, it doesn't remove the error code from the stack before issuing
the iret instruction; it therefore attempts to return to a garbage
address.  Since the segment part of this address is likely to be
invalid, a second general protection fault occurs.  This cycle
continues until we run out of stack space and triple fault.

Fix by reflecting all INTs down to real mode.  This actually reduces
the code size by four bytes (but increases the bss size by almost
2kB).

Reported-by: Brian Rak <dn@devicenull.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>