[nvptx] Fix Wimplicit-fallthrough in nvptx.c with -save-temps

When compiling nvptx.c using -save-temps, I ran into Wimplicit-fallthrough
warnings.

The fallthrough locations have been marked with a fallthrough comment, but
that doesn't work with -save-temps, something that has been filed as
PR78497.

Work around this by using gcc_fallthrough () in addition to the comment.

Tested by building target nvptx, copying nvptx.c compile line and adding
-save-temps.

gcc/ChangeLog:

2020-09-25  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx.c (nvptx_assemble_integer, nvptx_print_operand):
	Use gcc_fallthrough ().