x86: probe assembler capabilities via kconfig instead of makefile

Doing this probing inside of the Makefiles means we have a maze of
ifdefs inside the source code and child Makefiles that need to make
proper decisions on this too. Instead, we do it at Kconfig time, like
many other compiler and assembler options, which allows us to set up the
dependencies normally for full compilation units. In the process, the
ADX test changes to use %eax instead of %r10 so that it's valid in both
32-bit and 64-bit mode.

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

net/netfilter/Makefile

@@ -83,7 +83,7 @@ nf_tables-objs := nf_tables_core.o nf_tables_api.o nft_chain_filter.o \
 		  nft_set_pipapo.o
 
 ifdef CONFIG_X86_64
-ifneq (,$(findstring -DCONFIG_AS_AVX2=1,$(KBUILD_CFLAGS)))
+ifdef CONFIG_AS_AVX2
 nf_tables-objs += nft_set_pipapo_avx2.o
 endif
 endif