Fix compile on raspberry pi 4/A72

Game runs pretty close to fine, needed to use system libpng and haven't
attempted to use ffmpeg, however.

I'm seeing in the 50fps range in gameplay with no performance tweaks
but having set -mtune=cortex-a72. There's a lot of perf opportunities,
so consistent 60 is certainly within reason without too much pain.

src/archutils/Common/PthreadHelpers.cpp

@@ -244,6 +244,9 @@ bool GetThreadBacktraceContext( uint64_t ThreadID, BacktraceContext *ctx )
 	ctx->PC = (void *)ptrace( PTRACE_PEEKUSER, pid_t(ThreadID), (void *)(PT_NIP<<2), 0 );
 	if( errno )
 		return false;
+#elif defined(CPU_AARCH64)
+	// NYI
+	return false;
 #else
 #error GetThreadBacktraceContext: which arch?
 #endif