build-kernel.sh: Fix --savedefconfig for x86_64

Using --arch=x86_64 and --savedefconfig didn't work properly:

    mv: cannot move `defconfig' to `arch/x86_64/configs/x86_64_emu_defconfig': No such file or directory

Because the destination is under arch/x86, not arch/x86_64.

This patch fixes the issue.

Change-Id: Iebd49250cbcb1c0f4b3e0e096acea7ae5cafedd6
Merge "build-kernel.sh: Fix --savedefconfig for x86_64"