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>

arch/x86/crypto/blake2s-core.S

@@ -46,7 +46,6 @@ SIGMA2:
 #endif /* CONFIG_AS_AVX512 */
 
 .text
-#ifdef CONFIG_AS_SSSE3
 SYM_FUNC_START(blake2s_compress_ssse3)
 	testq		%rdx,%rdx
 	je		.Lendofloop
@@ -174,7 +173,6 @@ SYM_FUNC_START(blake2s_compress_ssse3)
 .Lendofloop:
 	ret
 SYM_FUNC_END(blake2s_compress_ssse3)
-#endif /* CONFIG_AS_SSSE3 */
 
 #ifdef CONFIG_AS_AVX512
 SYM_FUNC_START(blake2s_compress_avx512)