FEXCore: Fixes ever shrinking JIT code buffer

I accidentally replaced a couple usages of `AllocatedSize` with
`GetAllocatedSize()`. This resulted in a JIT buffer that ran out of
space would actually allocate a slightly smaller buffer each time, and
then it cascades downwards resulting in catastrophic performance.

Fix the use in `SharedCodeBufferManager.cpp` and `Core.cpp` which were
incorrect and renames the function to be more explicit.

FEXCore/Source/Interface/Core/SharedCodeBufferManager.h

@@ -71,8 +71,13 @@ struct CodeBuffer {
     return AllocatedSize - FEXCore::Utils::FEX_PAGE_SIZE;
   }
 
+  // Returns the full size of the buffer, including the guard page.
+  size_t TotalAllocationSize() const {
+    return AllocatedSize;
+  }
+
   // Returns the num of bytes currently allocated from the allocator.
-  size_t GetAllocatedSize() const {
+  size_t AllocatedSpaceUsed() const {
     return CodeBufferOffset - Ptr;
   }