arm: respect both __ARM_ARCH_6KZ__ and __ARM_ARCH_6ZK__ macros

arm: respect both __ARM_ARCH_6KZ__ and __ARM_ARCH_6ZK__ macros

__ARM_ARCH_6ZK__ is a gcc specific historical typo which may not be
defined by other compilers.

  https://gcc.gnu.org/ml/gcc-patches/2015-07/msg02237.html

To avoid unexpected results when building for ARMv6KZ with clang, the
correct form of the macro (ie 6KZ) needs to be tested. The incorrect
form of the macro (ie 6ZK) still needs to be tested for compatibility
with pre-2015 versions of gcc.