AArch64 broken due to CRC32

Attempting to build with an aarch64 gcc (4.9.2) that has __ARM_NEON but no __ARM_FEATURE_CRC32 breaks when attempting to include arm_acle.h as there is no such file.

Note that the block was not hit before due to Q_PROCESSOR_ARMV8 not getting defined due to another bug. See https://codereview.qt-project.org/#/c/159632/