ArmPkg/CpuDxe: unmask SErrors in DEBUG builds

SErrors (formerly called asynchronous aborts) are a distinct class of
exceptions that are not closely tied to the currently executing
instruction. Since execution may be able to proceed in such a condition,
this class of exception is masked by default, and software needs to unmask
it explicitly if it is prepared to handle such exceptions.

On DEBUG builds, we are well equipped to report the CPU context to the user
and it makes sense to report an SError as soon as it occurs rather than to
wait for the OS to take it when it unmasks them, especially since the current
arm64/Linux implementation simply panics in that case. So unmask them when
ArmCpuDxe loads.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

ArmPkg/Drivers/CpuDxe/Exception.c

@@ -62,6 +62,15 @@ InitializeExceptions (
     Status = Cpu->EnableInterrupt (Cpu);
   }
 
+  //
+  // On a DEBUG build, unmask SErrors so they are delivered right away rather
+  // than when the OS unmasks them. This gives us a better chance of figuring
+  // out the cause.
+  //
+  DEBUG_CODE (
+    ArmEnableAsynchronousAbort ();
+  );
+
   return Status;
 }