uname: switch machine to HW_MACHINE_ARCH

[pkubaj]

Summary

On powerpc64, powerpc64le and riscv64 some software wrongly assumes that
it runs on powerpc or riscv (32-bit).

Test Plan

make universe

[jhibbits]

If you're going by the output of 'uname -a' or 'uname -m', on FreeBSD you can see the MACHINE_ARCH with uname -p instead.

[imp]

I'm sure this would break things...
What software? We've had this "issue" for ~2 decades since powerpc grew 64 bit support...
Why now?

[imp]

In D35962#816293, @jhibbits wrote:

If you're going by the output of 'uname -a' or 'uname -m', on FreeBSD you can see the MACHINE_ARCH with uname -p instead.

Yea, this isn't even used for uname(1)...

[pkubaj]

One would be Perl's uname(): https://github.com/Perl/perl5/blob/blead/ext/POSIX/POSIX.xs#L3349. This causes e.g. OpenSSL's build system to be explicitly told what architecture to build (see security/openssl port).
Another would be Firefox's generation of default UA: https://searchfox.org/mozilla-central/source/netwerk/protocol/http/nsHttpHandler.cpp#931

I'm pretty sure there are others, it's just that powerpc and riscv are not that widely used. It also benefits armv*, since with this change either armv6 or armv7 should be printed, instead of plain arm

[pkubaj]

This doesn't change anything for amd64 and i386 and very little for aarch64 (aarch64 will be printed instead of arm64), so I'm not sure what breakage you think about