powerpc/64: move interrupt return asm to interrupt_64.S

The next patch would like to move interrupt return assembly code to a low
location before general text, so move it into its own file and include via
head_64.S

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210617155116.2167984-7-npiggin@gmail.com