x86: remove always-defined CONFIG_AS_SSSE3

CONFIG_AS_SSSE3 was introduced by commit 75aaf4c3e6 ("x86/raid6:
correctly check for assembler capabilities").

We raise the minimal supported binutils version from time to time.
The last bump was commit 1fb12b35e5 ("kbuild: Raise the minimum
required binutils version to 2.21").

I confirmed the code in $(call as-instr,...) can be assembled by the
binutils 2.21 assembler and also by LLVM integrated assembler.

Remove CONFIG_AS_SSSE3, which is always defined.

I added ifdef CONFIG_X86 to lib/raid6/algos.c to avoid link errors
on non-x86 architectures.

lib/raid6/algos.c is built not only for the kernel but also for
testing the library code from userspace. I added -DCONFIG_X86 to
lib/raid6/test/Makefile to cator to this usecase.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Ingo Molnar <mingo@kernel.org>

lib/raid6/algos.c

@@ -97,13 +97,13 @@ void (*raid6_datap_recov)(int, size_t, int, void **);
 EXPORT_SYMBOL_GPL(raid6_datap_recov);
 
 const struct raid6_recov_calls *const raid6_recov_algos[] = {
+#ifdef CONFIG_X86
 #ifdef CONFIG_AS_AVX512
 	&raid6_recov_avx512,
 #endif
 #ifdef CONFIG_AS_AVX2
 	&raid6_recov_avx2,
 #endif
-#ifdef CONFIG_AS_SSSE3
 	&raid6_recov_ssse3,
 #endif
 #ifdef CONFIG_S390