On amd64 adding DEFAULT_VERSIONS+=linux=c6 to /etc/make.conf made Linux
infrastructure ports (linux_base-c6 and linux-c6*) install i386 CentOS 6
packages. Using c6_64 instead of c6 made them install mixed amd64/i386
CentOS 6 packages and allowed Linux application ports to install amd64
binaries when available. The i386-only configuration was only useful for
users of x11/nvidia-driver* which didn't provide Linux amd64 emulation
until recently.

With x11/nvidia-driver* now supporting Linux amd64 there's no longer a
need for i386-only Linux on amd64. Let DEFAULT_VERSIONS+=linux=c6 make
Linux infrastructure ports install mixed amd64/i386 CentOS 6 packages,
like c6_64 before, and remove support for c6_64 as a value.

Replace LINUX_ARCH with ARCH everywhere because they are always the same
now.