[librm] Add support for installing a startup IPI handler

Application processors are started via INIT and SIPI interprocessor
interrupts: the INIT places the processor into a "wait for SIPI"
state, and the SIPI then starts the processor in real mode at a
page-aligned address derived from the SIPI vector number.

Add support for installing a real-mode SIPI handler that will switch
the CPU into protected mode with flat physical addressing, load
initial register contents, and then jump to the address of a
protected-mode SIPI handler.  No stack pointer is set up, to avoid the
need to allocate stack space for each available processor.

We use 32-bit physical addressing in order to minimise the changes
required for a 64-bit build.  The existing long mode transition code
relies on the existence of the stack, so we cannot easily switch the
application processor into long mode.  We could use 32-bit virtual
addressing, but this runtime environment does not currently exist
outside of librm.S itself in a 64-bit build, and using it would
complicate the implementation of the protected-mode SIPI handler.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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