implement GetSignalBacktraceContext on DARWIN

stepmania/src/archutils/Unix/Backtrace.cpp

@@ -413,6 +413,12 @@ void GetExceptionBacktraceContext( BacktraceContext *ctx, const ExceptionInforma
 	ctx->FramePtr = (void *) exception->registerImage->R1.lo;
 }
 
+void GetSignalBacktraceContext( BacktraceContext *ctx, const ucontext_t *uc )
+{
+	ctx->PC = (void *) uc->uc_mcontext->ss.srr0;
+	ctx->FramePtr = (void *) uc->uc_mcontext->ss.r1;
+}
+
 void InitializeBacktrace() { }
 
 void GetBacktrace( const void **buf, size_t size, const BacktraceContext *ctx )

stepmania/src/archutils/Unix/Backtrace.h

@@ -33,10 +33,8 @@ bool GetThreadBacktraceContext( int ThreadID, BacktraceContext *ctx );
 
 /* Set up a BacktraceContext to get a backtrace after receiving a signal, given
  * a ucontext_t (see sigaction(2)).  (This interface is UNIX-specific.) */
-#if !defined(DARWIN)
 #include <ucontext.h>
 void GetSignalBacktraceContext( BacktraceContext *ctx, const ucontext_t *uc );
-#endif
 
 #if defined(DARWIN)
 #include <MachineExceptions.h>