gcc: fix nvptx conflict (#22214)
gcc: fix nvptx conflict (#22214)

In the past, we only had the binutils variant, which included the
bootstrapping flag. Now that we have a separate bootstrap variant, fix
the nvptx conflict accordingly.
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack