arm64/neon: Disable -Wincompatible-pointer-types when building with Clang.

After commit cc9f834 ("arm64: crypto: add NEON accelerated XOR
implementation"), Clang builds for arm64 started failing with the
following error message.

arch/arm64/lib/xor-neon.c:58:28: error: incompatible pointer types
assigning to 'const unsigned long *' from 'uint64_t *' (aka 'unsigned
long long *') [-Werror,-Wincompatible-pointer-types]
               v3 = veorq_u64(vld1q_u64(dp1 +  6), vld1q_u64(dp2 + 6));
                                        ^~~~~~~~
/usr/lib/llvm-9/lib/clang/9.0.0/include/arm_neon.h:7538:47: note:
expanded from macro 'vld1q_u64'
 __ret = (uint64x2_t) __builtin_neon_vld1q_v(__p0, 51); \
                                             ^~~~

There has been quite a bit of debate and triage that has gone into
figuring out what the proper fix is, viewable at the link below, which
is still ongoing. Ard suggested disabling this warning with Clang with a
pragma so no neon code will have this type of error. While this is not
at all an ideal solution, this build error is the only thing preventing
KernelCI from having successful arm64 defconfig and allmodconfig builds
on linux-next. Getting continuous integration running is more important
so new warnings/errors or boot failures can be caught and fixed quickly.

Link: #283
Change-Id: I278fe2e64c37b114052fd75d09757411385a0f58
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>