arch-x86,arch-arm: Remove static variables in decoders (#1643)

There were a number of variables in the arm and x86 decoders that are
static (e.g., the decode cache). It's a bit interesting that this
doesn't cause problems with multiple cores since each core has its own
decoder.

However, this causes segfaults if you run different cores on different
*host* threads. We are experimenting with running gem5 with multiple
host thread (i.e., in parallel), and removing these static variables
resolves the segfault.

This change also adds const to any other static variables to ensure that
they cannot be modified.

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>

src/arch/arm/decoder.hh

@@ -94,7 +94,7 @@ class Decoder : public InstDecoder
     enums::DecoderFlavor decoderFlavor;
 
     /// A cache of decoded instruction objects.
-    static GenericISA::BasicDecodeCache<Decoder, ExtMachInst> defaultCache;
+    GenericISA::BasicDecodeCache<Decoder, ExtMachInst> defaultCache;
     friend class GenericISA::BasicDecodeCache<Decoder, ExtMachInst>;
 
     /**