Building with MinGW-w64 results in ENABLE_EXECUTABLE_ALLOCATOR_FIXED being defined to 1 in Platform.h. Compiling javascriptcore/JavaScriptCore/jit/ExecutableAllocatorFixedVMPool.cpp results in a compile error because sys/mman.h is not available when using MinGW. Platform.h needs "#if CPU(X86_64) || PLATFORM(IOS)" changed to "#if (CPU(X86_64) || PLATFORM(IOS)) && !COMPILER(MINGW64)".
This seems to be already fixed. I was looking at the wrong version.