target-arm: A64: Implement DC ZVA

Implement the DC ZVA instruction, which clears a block of memory.
The fast path obtains a pointer to the underlying RAM via the TCG TLB
data structure so we can do a direct memset(), with fallback to a
simple byte-store loop in the slow path.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Acked-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>

target-arm/cpu64.c

@@ -46,6 +46,7 @@ static void aarch64_any_initfn(Object *obj)
     set_feature(&cpu->env, ARM_FEATURE_V7MP);
     set_feature(&cpu->env, ARM_FEATURE_AARCH64);
     cpu->ctr = 0x80030003; /* 32 byte I and D cacheline size, VIPT icache */
+    cpu->dcz_blocksize = 7; /*  512 bytes */
 }
 #endif