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
diff --git a/host/cros_choose_profile b/host/cros_choose_profile
index 0397524..be433be 100755
--- a/host/cros_choose_profile
+++ b/host/cros_choose_profile
@@ -85,6 +85,9 @@
   *86)
     ARCH="x86"
     ;;
+  x86_64)
+    ARCH="amd64"
+    ;;
   *)
     error "Unable to determine ARCH from toolchain: ${BOARD_VARIANT}"
     exit 1