gcc package: skip nvptx_install for gcc~nvptx (#10672)
gcc package: skip nvptx_install for gcc~nvptx (#10672)

The nvptx installation support phase (added in 040b658) was always
running for gcc regardless of whether +nvptx was enabled. This
removes the "@when" syntax (which apparently is not working) and
adds a check inside of the "nvptx_install" function to bail out if
+nvptx is not enabled.
iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,iulian787/spack