[nvptx] Build libraries with mptx=3.1

In gcc-5 to gcc-11, the ptx isa version was 3.1.

On trunk, the default is now 6.0, which is also what will be the value in
the libraries.

Consequently, there may be setups with an older driver that worked with
gcc-11, but will become unsupported with gcc-12.

Fix this by building the libraries with mptx=3.1.

After this, setups with an older driver still won't work out of the box
with gcc-12, because the default ptx isa version has changed, but should work
after specifying mptx=3.1.

gcc/ChangeLog:

2022-03-03  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/t-nvptx (MULTILIB_EXTRA_OPTS): Add mptx=3.1.