arch: Templatize the BasicDecodeCache.

While the arch/generic directory is in arch/, it still shouldn't assume
any particular ISA. This change templatizes away the ISA specific types
so it can be used in multiple ISAs at a time.

Change-Id: I1abb4f5081a0a25f743be786ad8e7e3d55cfc67a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40097
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/generic/SConscript

@@ -52,5 +52,4 @@ DebugFlag('TLB')
 if env['TARGET_ISA'] == 'null':
     Return()
 
-Source('decode_cache.cc')
 Source('decoder.cc')