Use __ARM_ARCH_6ZK__ instead of __ARM_ARCH_6KZ__. The architecture is
technically called ARMv6KZ, but GCC has always called it 6ZK.

Clang defines __ARM_ARCH_6K__ for this target, and is unaffected.

BUG=v8:3978
LOG=N

Review URL: https://codereview.chromium.org/1145973004

Cr-Commit-Position: refs/heads/master@{#28709}