platforms/i386: report Pentium Pro as minimum supported x86 processor

[emaste]

Summary

As of FreeBSD 13.0[1] we default to i686 for 32-bit x86. It is still possible to compile FreeBSD/i386 for i486 or i586 but I'm not convinced it's worth mentioning here, as the architecture is on the path to removal.

(I could perhaps add a statement like "It should be possible to build a custom FreeBSD/i386 kernel and userland for i486 or i586, but this is not regularly tested by the FreeBSD project and may not work.")

[1] https://www.freebsd.org/releases/13.0R/relnotes/

[emaste]

Prompted in part by comments from https://forums.freebsd.org/threads/heads-up-freebsd-is-stopping-all-32-bit-hardware-support-except-armv7.91811/page-3

[khorben_defora.org]

Would it be more clear or accurate to mention the "Pentium MMX" instead or in addition to the Pentium Pro?
It was much more common and known to the public than the Pentium Pro.

[imp]

In D45734#1043144, @khorben_defora.org wrote:

Would it be more clear or accurate to mention the "Pentium MMX" instead or in addition to the Pentium Pro?
It was much more common and known to the public than the Pentium Pro.

Pentium MMX is still i586. Pentium Pro and Pentium II were the first i686 CPU, A which id our current minimum

[imp]

I haven't heard of anybody building i586 or i486 in a while. That suggests that it isn't common / interesting enough to highlight here.

Maybe say Pentium II instead. It might be less confusing.