'gcc/config/nvptx/t-nvptx': Don't use the 'shell' function of 'make'

'gcc/config/nvptx/t-nvptx': Don't use the 'shell' function of 'make'

The exit status of the command invoked in a 'Makefile' via '$(shell [...])'
effectively gets discarded (unless explicitly checking the GNU Make 4.2+
'.SHELLSTATUS' variable or jumping through other hoops).  In order to be able
to catch errors in what the 'shell' function invokes, let's make things
explicit: similar to how 'gcc/config/avr/t-avr' is doing with 't-multilib-avr',
for example.

	gcc/
	* config/nvptx/t-nvptx (multilib_matches): Don't use the 'shell'
	function of 'make'.
	* config/nvptx/gen-multilib-matches.sh: Adjust.