[nvptx] Restore default to sm_30

[nvptx] Restore default to sm_30

With commit 07667c911b1 ("[nvptx] Build libraries with misa=sm_30") the
intention was that the sm_xx for all libraries was switched back to sm_30
using MULTILIB_EXTRA_OPTS, without changing the default sm_35.

Testing on an sm_30 board revealed that still some libs were build with sm_35,
so fix this by switching back to default sm_30.

Tested on nvptx.

gcc/ChangeLog:

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

	PR target/104758
	* config/nvptx/nvptx.opt (misa): Set default to sm_30.
	* config/nvptx/t-nvptx (MULTILIB_EXTRA_OPTS): Remove misa=sm_30.