Refactor MachineContext.h to reduce ifdefs elsewhere.

Also fixes Windows on ARM64 (at least the build).

Core/MemMap.cpp

@@ -29,24 +29,14 @@
 #include "Common/MemArena.h"
 #include "Common/ChunkFile.h"
 
-#ifdef __FreeBSD__
-#include <signal.h>
-#endif
-#ifndef _WIN32
-#include <unistd.h>  // Needed for _POSIX_VERSION
-#endif
+#include "Common/MachineContext.h"
 
 #if PPSSPP_ARCH(AMD64) || PPSSPP_ARCH(X86)
-#include "Common/MachineContext.h"
 #include "Common/x64Analyzer.h"
-#elif PPSSPP_ARCH(ARM64) && !PPSSPP_PLATFORM(IOS)
+#elif PPSSPP_ARCH(ARM64)
 #include "Core/Util/DisArm64.h"
-typedef sigcontext SContext;
-#define CTX_PC pc
-#elif PPSSPP_ARCH(ARM) && !PPSSPP_PLATFORM(IOS)
+#elif PPSSPP_ARCH(ARM)
 #include "ext/disarm.h"
-typedef sigcontext SContext;
-#define CTX_PC arm_pc
 #endif
 
 #include "Core/MemMap.h"