powerpc/85xx: Fix math emulation exception

Booting mpc85xx_defconfig kernel on QEMU leads to:

Bad trap at PC: fe9bab0, SR: 2d000, vector=800
awk[82]: unhandled trap (5) at 0 nip fe9bab0 lr fe9e01c code 5 in libc-2.27.so[fe5a000+17a000]
awk[82]: code: 3aa00000 3a800010 4bffe03c 9421fff0 7ca62b78 38a00000 93c10008 83c10008
awk[82]: code: 38210010 4bffdec8 9421ffc0 7c0802a6 <fc00048e> d8010008 4815190d 93810030
Trace/breakpoint trap
WARNING: no useful console

This is because allthough CONFIG_MATH_EMULATION is selected,
Exception 800 calls unknown_exception().

Call emulation_assist_interrupt() instead.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/066caa6d9480365da9b8ed83692d7101e10ac5f8.1695657339.git.christophe.leroy@csgroup.eu