[testsuite, nvptx] Add effective target sync_int_long_stack

[testsuite, nvptx] Add effective target sync_int_long_stack

The nvptx target currently doesn't support effective target sync_int_long,
although it has support for 32-bit and 64-bit atomic.

When enabling sync_int_long for nvptx, we run into a failure in
gcc.dg/pr86314.c:
...
 nvptx-run: error getting kernel result: operation not supported on \
   global/shared address space
...
due to a ptx restriction:  accesses to local memory are illegal, and the
test-case does an atomic operation on a stack address, which is mapped to
local memory.

Fix this by adding a target sync_int_long_stack, wich returns false for nvptx,
which can be used to mark test-cases that require sync_int_long support for
stack addresses.

Build on nvptx and tested with make check-gcc.

gcc/testsuite/ChangeLog:

	PR target/96494
	* lib/target-supports.exp (check_effective_target_sync_int_long):
	Return 1 for nvptx.
	(check_effective_target_sync_int_long_stack): New proc.
	* gcc.dg/pr86314.c: Require effective target sync_int_long_stack.