gba: reset prefetch buffer on ROM accesses from CPU (#1377)

According to NanoBoyAdvance author fleroviux, the prefetch buffer should
be reset on any ROM accesses from the CPU or DMA. This PR implements
this behaviour for CPU ROM accesses. This allows ares to pass 1760/2020
of the timing tests in the mGBA test suite (up from 1608/2020).

Allowing DMA ROM accesses to clear the prefetch buffer seems to worsen
timings at the moment, possibly due to some other timing issues that
also come into effect in these situations. I've left comments to make a
note of the issue, so that resetting the prefetch buffer on ROM accesses
from DMA can be implemented once it becomes beneficial to do so.

ares/gba/cpu/bus.cpp

@@ -15,7 +15,7 @@ auto CPU::get(u32 mode, n32 address) -> n32 {
       word = prefetchRead();
       if(mode & Word) word |= prefetchRead() << 16;
     } else {
-      if(!context.dmaActive) prefetchWait();
+      if(!context.dmaActive) prefetchReset();  //todo: Prefetch buffer should also be reset when DMA accesses ROM
       step(clocks - 1);
       word = cartridge.read(mode, address);
       step(1);
@@ -42,7 +42,7 @@ auto CPU::set(u32 mode, n32 address, n32 word) -> void {
   if(address >= 0x1000'0000) {
     prefetchStep(clocks);
   } else if(address & 0x0800'0000) {
-    if(!context.dmaActive) prefetchWait();
+    if(!context.dmaActive) prefetchReset();  //todo: Prefetch buffer should also be reset when DMA accesses ROM
     step(clocks);
     cartridge.write(mode, address, word);
   } else {

ares/gba/cpu/cpu.hpp

@@ -52,7 +52,7 @@ struct CPU : ARM7TDMI, Thread, IO {
   //prefetch.cpp
   auto prefetchSync(n32 address) -> void;
   auto prefetchStep(u32 clocks) -> void;
-  auto prefetchWait() -> void;
+  auto prefetchReset() -> void;
   auto prefetchRead() -> n16;
 
   //bus.cpp

ares/gba/cpu/prefetch.cpp

@@ -18,11 +18,9 @@ auto CPU::prefetchStep(u32 clocks) -> void {
   }
 }
 
-auto CPU::prefetchWait() -> void {
-  if(!wait.prefetch || context.dmaActive || prefetch.full()) return;
-
-  prefetchStep(prefetch.wait);
-  prefetch.wait = _wait(Half | Nonsequential, prefetch.load);
+auto CPU::prefetchReset() -> void {
+  prefetch.addr = 0;
+  prefetch.load = 0;
 }
 
 auto CPU::prefetchRead() -> n16 {