ArmPkg/CpuDxe: ignore attribute changes during SyncCacheConfig()

To prevent the initial MMU->GCD memory space map synchronization from
stripping permissions attributes [which we cannot use in the GCD memory
space map, unfortunately], implement the same approach as x86, and ignore
SetMemoryAttributes() calls during the time SyncCacheConfig() is in
progress. This is a horrible hack, but is currently the only way we can
implement strict permissions on arbitrary memory regions [as opposed to
PE/COFF text/data sections only]

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

ArmPkg/Drivers/CpuDxe/CpuDxe.h

@@ -37,6 +37,7 @@
 #include <Protocol/DebugSupportPeriodicCallback.h>
 #include <Protocol/LoadedImage.h>
 
+extern BOOLEAN mIsFlushingGCD;
 
 /**
   This function registers and enables the handler specified by InterruptHandler for a processor