Switch atmosphere's build target to C++20. (#952)

* ams: update to build with gcc10/c++20

* remove mno-outline-atomics

* ams: take care of most TODO C++20s

* fusee/sept: update for gcc10

* whoosh, your code now uses pre-compiled headers

* make: dependency fixes

fusee/fusee-primary/src/fs_utils.h

@@ -13,7 +13,7 @@
  * You should have received a copy of the GNU General Public License
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
- 
+
 #ifndef FUSEE_FS_UTILS_H
 #define FUSEE_FS_UTILS_H
 
@@ -23,8 +23,8 @@
 #include "sdmmc/sdmmc.h"
 #include "utils.h"
 
-sdmmc_t g_sd_sdmmc;
-sdmmc_device_t g_sd_device;
+extern sdmmc_t g_sd_sdmmc;
+extern sdmmc_device_t g_sd_device;
 
 bool mount_sd(void);
 void unmount_sd(void);