adding x86_64 to TC_ARCH
adding x86_64 to TC_ARCH

Change-Id: Iae09e86bfbe8444e14a7e63f9e50d171a67618cf

BUG=13363
TEST=Manually built the chroot with this change.

Review URL: http://codereview.chromium.org/6849001

mgireesh05/dev-util,coreos/dev-util,coreos/dev-util,coreos/dev-util,marineam/coreos-dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,mgireesh05/dev-util