Add support for native Solaris compiler on x86.

Original patch by Ginn Chen <ginn.chen@oracle.com> against libvpx
 v0.9.0.
I've forward-ported it to the current version (which mostly
 involved removing hunks that were no longer relevant), since I've
 given up on getting Ginn to submit this upstream himself.

Change-Id: I403c757c831c78d820ebcfe417e717b470a1d022