powerpc: Update CPUTYPE options in make.conf

[ivy]

Summary

Document the supported PPC/POWER CPUTYPEs in the example make.conf.

Update bsd.cpu.mk to handle CPUs newer than POWER9, and remove
32-bit CPUTYPEs since we no longer support those at all.

[jhibbits]

Isn't make.conf also read for userspace builds and ports? I think we still support 32-bit userland. Would there be a corresponding change to remove the i386 bits as well?

[adrian]

has the 32 bit ppc kernel code gone away yet?

[ivy]

In D54257#1239773, @jhibbits wrote:

Isn't make.conf also read for userspace builds and ports? I think we still support 32-bit userland. Would there be a corresponding change to remove the i386 bits as well?

my understanding is that CPUTYPE is orthogonal to word length and specifies the physical CPU type, since it controls things like instruction scheduling. since we only boot on 64-bit CPUs, there's no reason to ever specify a 32-bit-only CPU in CPUTYPE: when compiling a 32-bit application on a PPC970, you would use cc -m32 -mcpu=970, which will generate 32-bit code even though the 970 is a 64-bit CPU.

so we should probably remove i486 as well, yes.

In D54257#1239774, @adrian wrote:

has the 32 bit ppc kernel code gone away yet?

no, but it is no longer supported, so we don't need to support it. it would be different if it had been downgraded to tier 3 rather than becoming entirely unsupported.