powerpc/64s/exception: machine check remove machine_check_pSeries_0 branch

This label has only one caller, so unwind the branch and move it
inline. The location of the comment is adjusted to match similar
one in system reset.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190802105709.27696-7-npiggin@gmail.com