nvptx: stack size limits are relevant for execution only

nvptx: stack size limits are relevant for execution only

For non-'dg-do run' test cases, that means: big 'dg-require-stack-size' need
not be UNSUPPORTED (and indeed now do all PASS), 'dg-add-options stack_size'
need not define (and thus limit) 'STACK_SIZE' (and still do all PASS).

Re "Find 'dg-do-what' in an outer frame", currently (sources not completely
clean, though), we've got:

    $ git grep -F 'check_effective_target_stack_size: found dg-do-what at level ' -- build-gcc/\*.log | sort | uniq -c
          6 build-gcc/gcc/testsuite/gcc/gcc.log:check_effective_target_stack_size: found dg-do-what at level 2
        267 build-gcc/gcc/testsuite/gcc/gcc.log:check_effective_target_stack_size: found dg-do-what at level 3
        239 build-gcc/gcc/testsuite/gcc/gcc.log:check_effective_target_stack_size: found dg-do-what at level 4

	gcc/testsuite/
	* lib/target-supports.exp (check_effective_target_stack_size): For
	nvptx target, stack size limits are relevant for execution only.
	gcc/
	* doc/sourcebuild.texi (stack_size): Update.