[nvptx] Add multilib mptx=3.1

[nvptx] Add multilib mptx=3.1

With commit 5b5e456f018 ("[nvptx] Build libraries with mptx=3.1") the
intention was that the ptx isa version for all libraries was switched back to
3.1 using MULTILIB_EXTRA_OPTS, without changing the default 6.0.

Further testing revealed that this is not the case, and some libs were still
build with 6.0.

Fix this by introducing an mptx=3.1 multilib.

Adding a multilib should be avoided if possible, because it adds build time.
But I think it's a reasonable trade-off.  With --disable-multilib, the default
lib with misa=sm_30 and mptx=6.0 should be usable in most scenarios.  With
--enable-multilib, we can enable older drivers, as well as generate code
similar to how that was done in previous gcc releases, which is very useful.

Tested on nvptx.

gcc/ChangeLog:

2022-03-07  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/t-nvptx (MULTILIB_EXTRA_OPTS): Move mptx=3.1 ...
	(MULTILIB_OPTIONS): ... here.