buildbot-build.sh: don't try to copy nonexistent 32-bit libraries.

Test: in aosp-main:
    $ lunch aosp_riscv64-userdebug
    $ echo $ANDROID_PRODUCT_OUT/system/lib
    .../out/target/product/generic_riscv64/system/lib
    $ run 001-HelloWorld on a riscv64 VM

Test: on aosp-master-art:
    $ lunch riscv64
    $ echo $ANDROID_PRODUCT_OUT/system/lib
    .../out/target/product/riscv64/system/lib
    $ run 001-HelloWorld on a riscv64 VM

Change-Id: I90ca549cae4e3c84ba89827cb9e0ec9d3104f2e3