Adding a MACHINE_ARCH note

[Konstantin Belousov]

On Wed, Jul 10, 2013 at 06:43:22PM +1000, Bruce Evans wrote:
> 
> ISTR reading a claim that pure 64-bit mode was mainly a marketing ploy
> by AMD.  It distinguished them from Intel and let them get to market
> faster.  The result was a marketing win but a not so good instruction
> set architecture.  This was contrasted with sparc64 (Sun?), where
> 32-bit applications remained the default and the differences were
> smaller (no mode switch?).

Sparcv8 did not have an issue with the very limited size of the register
file.  They did not need to change instruction encoding to allow bigger
set of the registers.  Amd had to change encoding, and indeed I remember
reading somewhere that they decided to not allow the REX prefix for
compatibility mode to push the "full" 64bit tagline.

Solaris port to amd64 is very similar to sparcv9: almost all usermode
binaries are 32 bit, libraries are supplied in both 32 and 64 bit
versions, compiler defaults to -m32. Only several binaries are 64 bit,
most notable are procfs tools and debugger.

There is no reason why such approach could not been taken for the
FreeBSD port, except convenience and avoiding the work for properly
support all kernel interfaces in compat 32bit and 64bit variants from
the beginning.

We still have almost no compat32 for the management interfaces, esp.
network. cc -m32 works in HEAD. I believe that multiarch for ports is
not even thought about, would be very happy to be wrong there.
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