[nvptx] Build libraries with misa=sm_30

[nvptx] Build libraries with misa=sm_30

In gcc-11, when  specifying -misa=sm_30, an executable may still contain sm_35
code (due to libraries being built with the default -misa=sm_35), so it won't
run on an sm_30 board.

Fix this by building libraries with sm_30, as was the case in gcc-5 to gcc-10.

gcc/ChangeLog:

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

	PR target/104758
	* config/nvptx/t-nvptx (MULTILIB_EXTRA_OPTS): Add misa=sm_30.