With GCC 12, corenet64_smp_defconfig leads to the following build errors:

  CC      arch/powerpc/kernel/irq.o
{standard input}: Assembler messages:
{standard input}:3616: Error: unrecognized opcode: `wrteei'
{standard input}:5689: Error: unrecognized opcode: `wrteei'
  CC      arch/powerpc/kernel/pmc.o
{standard input}: Assembler messages:
{standard input}:42: Error: unrecognized opcode: `mfpmr'
{standard input}:53: Error: unrecognized opcode: `mtpmr'
  CC      arch/powerpc/kernel/io.o
{standard input}: Assembler messages:
{standard input}:376: Error: unrecognized opcode: `mbar'
...
  CC      arch/powerpc/mm/nohash/book3e_hugetlbpage.o
{standard input}: Assembler messages:
{standard input}:291: Error: unrecognized opcode: `tlbsx'
{standard input}:482: Error: unrecognized opcode: `tlbwe'
{standard input}:608: Error: unrecognized opcode: `lbarx'
{standard input}:608: Error: unrecognized opcode: `stbcx.'

-mpcu=powerpc64 cannot be used anymore for book3e, it must be a booke CPU.

But then we get:

  CC      arch/powerpc/lib/xor_vmx.o
cc1: error: AltiVec not supported in this target

Altivec is not supported with -mcpu=e5500 so don't allow selection
of altivec when e5500 is selected.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/77255a5a957967723b84d0356d9e5fb21569f4e8.1657549153.git.christophe.leroy@csgroup.eu