solaris: build 64-bit binaries for sunx86_51x by default

solaris: build 64-bit binaries for sunx86_51x by default

Build 64-bit binaries on solaris x86 version 10 or better
build systems running in 64-bit mode.

Builders may set the the ARCHFLAGS environment variable
to "-m32" before running configure to build 32-bit binaries
on a 64-bit build host.

Change-Id: I27558a2c7fe9872cf30889ca57e778985a3ff461
Reviewed-on: http://gerrit.openafs.org/5450
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>