powerpc/64s: Prepare to handle data interrupts vs d-side MCE reentrancy

A subsequent fix for data interrupts (those that set DAR / DSISR)
requires some interrupt macros to be open-coded, and also requires
the 0x300 interrupt handler to be moved out-of-line.

This patch does that without changing behaviour, which makes the later
fix a smaller change.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>