crypto: arm/xor - make vectorized C code Clang-friendly

The ARM version of the accelerated XOR routines are simply the 8-way C
routines passed through the auto-vectorizer with SIMD codegen enabled.
This used to require GCC version 4.6 at least, but given that 5.1 is now
the baseline, this check is no longer necessary, and actually
misidentifies Clang as GCC < 4.6 as Clang defines the GCC major/minor as
well, but makes no attempt at doing this in a way that conveys feature
parity with a certain version of GCC (which would not be a great idea in
the first place).

So let's drop the version check, and make the auto-vectorize pragma
(which is based on a GCC-specific command line option) GCC-only. Since
Clang performs SIMD auto-vectorization by default at -O2, no pragma is
necessary here.

Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/496
Link: https://github.com/ClangBuiltLinux/linux/issues/503


Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>