gdb: disable python when python is not available.

This cherry-picks from https://gerrit.chromium.org/gerrit/65026
BUG=chromium:248605
TEST=gdb in chroot no regression and works outside chroot.
     cbuildbot lumpy passes.
     run bvt on lumpy, passes 96/102, the same as the vanilla image.

Change-Id: I87fe9aade149b4d329dcce14a4e44dd1b64b1c85
Reviewed-on: https://chromium-review.googlesource.com/171457
Reviewed-by: Caroline Tice <cmtice@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
8 files changed