fix win32 braindead _access()

The Windows library call `_access()` styled after POSIX `access(2)`
only allows combinations of `R_OK` and `W_OK` and returns `EINVAL`
otherwise.

cf. https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/access-waccess?view=msvc-170

v2: move to platform-specific header file
v3: just strip X_OK (@skullernet)

inc/shared/platform.h

@@ -64,7 +64,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #define os_stat(p, s)       _stat64(p, s)
 #define os_fstat(f, s)      _fstat64(f, s)
 #define os_fileno(f)        _fileno(f)
-#define os_access(p, m)     _access(p, m)
+#define os_access(p, m)     _access(p, (m) & ~X_OK)
 #define Q_ISREG(m)          (((m) & _S_IFMT) == _S_IFREG)
 #define Q_ISDIR(m)          (((m) & _S_IFMT) == _S_IFDIR)
 #define Q_STATBUF           struct _stat64