Silence implicit-fallthrough warnings

Used [[fallthrough]]; to mark affected areas. Such keyword should be
available since C++11

src/apu/Gb_Apu.cpp

@@ -218,6 +218,7 @@ void Gb_Apu::run_until_( blip_time_t end_time )
 		case 6:
 			// 128 Hz
 			square1.clock_sweep();
+			[[fallthrough]];
 		case 0:
 		case 4:
 			// 256 Hz

src/gba/Cheats.cpp

@@ -1352,6 +1352,7 @@ void cheatsDelete(int number, bool restore)
                 } else {
                     CPUWriteMemory(cheatsList[x].address, cheatsList[x].oldValue);
                 }
+                [[fallthrough]];
             case GSA_16_BIT_ROM_PATCH:
                 if (cheatsList[x].status & 1) {
                     cheatsList[x].status &= ~1;

src/gba/GBA.cpp

@@ -2250,7 +2250,8 @@ void CPUSoftwareInterrupt(int comment)
         break;
     case 0x2A:
         BIOS_SndDriverJmpTableCopy();
-    // let it go, because we don't really emulate this function
+        // let it go, because we don't really emulate this function
+        [[fallthrough]];
     default:
 #ifdef GBA_LOGGING
         if (systemVerbose & VERBOSE_SWI) {

src/gba/GBAinline.h

@@ -131,7 +131,7 @@ static inline uint32_t CPUReadMemory(uint32_t address)
             value = flashRead(address) * 0x01010101;
             break;
         }
-    // default
+        [[fallthrough]];
     default:
     unreadable:
 #ifdef GBA_LOGGING
@@ -281,7 +281,7 @@ static inline uint32_t CPUReadHalfWord(uint32_t address)
             value = flashRead(address) * 0x0101;
             break;
         }
-    // default
+        [[fallthrough]];
     default:
     unreadable:
         if (cpuDmaRunning|| ((reg[15].I - cpuDmaPC) == (armState ? 4 : 2))) {
@@ -407,7 +407,7 @@ static inline uint8_t CPUReadByte(uint32_t address)
         case 0x8500:
             return systemGetSensorY() >> 8;
         }
-    // default
+        [[fallthrough]];
     default:
     unreadable:
 #ifdef GBA_LOGGING