PR libstdc++/85632 fix wraparound in filesystem::space

PR libstdc++/85632 fix wraparound in filesystem::space

On 32-bit targets any values over 4GB would wrap and produce the wrong
result.

	PR libstdc++/85632 use uintmax_t for arithmetic
	* src/filesystem/ops.cc (experimental::filesystem::space): Perform
	arithmetic in result type.
	* src/filesystem/std-ops.cc (filesystem::space): Likewise.
	* testsuite/27_io/filesystem/operations/space.cc: Check total capacity
	is greater than free space.
	* testsuite/experimental/filesystem/operations/space.cc: New.

From-SVN: r259901