x86: add comments about the binutils version to support code in as-instr

We raise the minimal supported binutils version from time to time.
The last bump was commit 1fb12b35 ("kbuild: Raise the minimum
required binutils version to 2.21").

We have these as-instr tests because binutils 2.21 does not support
them.

When we bump the binutils version next time, this will be a good
hint to find out which one can be dropped.

As for the Clang/LLVM builds, we require very new LLVM version,
so the LLVM integrated assembler supports all of them.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>

arch/x86/Kconfig.assembler

@@ -3,15 +3,25 @@
 
 config AS_AVX2
 	def_bool $(as-instr,vpbroadcastb %xmm0$(comma)%ymm1)
+	help
+	  Supported by binutils >= 2.22 and LLVM integrated assembler
 
 config AS_AVX512
 	def_bool $(as-instr,vpmovm2b %k1$(comma)%zmm5)
+	help
+	  Supported by binutils >= 2.25 and LLVM integrated assembler
 
 config AS_SHA1_NI
 	def_bool $(as-instr,sha1msg1 %xmm0$(comma)%xmm1)
+	help
+	  Supported by binutils >= 2.24 and LLVM integrated assembler
 
 config AS_SHA256_NI
 	def_bool $(as-instr,sha256msg1 %xmm0$(comma)%xmm1)
+	help
+	  Supported by binutils >= 2.24 and LLVM integrated assembler
 
 config AS_ADX
 	def_bool $(as-instr,adox %eax$(comma)%eax)
+	help
+	  Supported by binutils >= 2.23 and LLVM integrated assembler