torture: Record TORTURE_KCONFIG_GDB_ARG in qemu-cmd

When re-running old rcutorture builds, if the original run involved
gdb, the re-run also needs to do so.  This commit therefore records the
TORTURE_KCONFIG_GDB_ARG environment variable into the qemu-cmd file so
that the re-run can access it.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>