Do not force "-static" for aarch64 iOS kernel/kext assembly code.

Somehow a check for aarch64 was added to the Darwin toolchain's
isKernelStatic function as part of the initial commit for Apple's
arm64 target (r205100). That check was not in any of Apple's internal
code and no one here knows where it came from. It has been harmless
because "-static" does not change much, if anything, for arm64 iOS code,
but it makes no sense to keep this check.

llvm-svn: 228673