libstdc++: Use correct error category for Windows error codes

libstdc++: Use correct error category for Windows error codes

When the result of GetLastError() is stored in a std::error_code it
should use std::system_category(), not std::generic_category() that is
used for POSIX errno values.

libstdc++-v3/ChangeLog:

	* src/c++17/fs_ops.cc (fs::create_hard_link, fs::equivalent)
	(fs::remove): Use std::system_category() for error codes from
	GetLastError().
	* src/filesystem/ops.cc (fs::create_hard_link, fs::remove):
	Likewise.