nvptx: Tag '-misa=[...]', '-mptx=[...]' as 'Negative' of themselves [PR117916]

nvptx: Tag '-misa=[...]', '-mptx=[...]' as 'Negative' of themselves [PR117916]

This issue is similar to what a year ago I resolved for GCN in PR112669
"GCN: wrong 'LIBRARY_PATH' in presence of several different '-march=[...]' flags".

Given the current standard nvptx configuration, we get:

    $ build-gcc-offload-nvptx-none/gcc/xgcc -print-multi-directory -mptx=6.3
    .
    $ build-gcc-offload-nvptx-none/gcc/xgcc -print-multi-directory -mptx=3.1
    mptx-3.1

... as expected.  The following, however, is not:

    $ build-gcc-offload-nvptx-none/gcc/xgcc -print-multi-directory -mptx=3.1 -mptx=6.3
    mptx-3.1

This should print '.'.

Or, in a '--with-arch=sm_70' configuration:

    $ build-gcc-offload-nvptx-none/gcc/xgcc -print-multi-directory -misa=sm_70
    .
    $ build-gcc-offload-nvptx-none/gcc/xgcc -print-multi-directory -misa=sm_30
    misa-sm_30

... as expected.  The following, however, are not:

    $ build-gcc-offload-nvptx-none/gcc/xgcc -print-multi-directory -misa=sm_30 -misa=sm_70
    misa-sm_30
    $ build-gcc-offload-nvptx-none/gcc/xgcc -print-multi-directory -misa=sm_30 -march=sm_70
    misa-sm_30
    $ build-gcc-offload-nvptx-none/gcc/xgcc -print-multi-directory -march=sm_30 -march=sm_70
    misa-sm_30
    $ build-gcc-offload-nvptx-none/gcc/xgcc -print-multi-directory -march=sm_30 -misa=sm_70
    misa-sm_30

These should all print '.'.

Even worse:

    $ build-gcc-offload-nvptx-none/gcc/xgcc -print-multi-directory -mgomp -mptx=3.1 -mptx=_
    .

This should print 'mgomp'.  Otherwise, for OpenMP offloading compilation
the wrong (non-'mgomp') multilib is linked in ('.'), and linking fails
due to 'unresolved symbol __nvptx_uni'.

	PR target/117916
	gcc/
	* config/nvptx/nvptx.opt (misa=, mptx=): Tag as 'Negative' of
	themselves.