arch/Config.in.x86: indicate how the CPU variants are ordered

They are loosely ordered according to the ordering of the gcc
documentation. It is not entirely correct as the generic x86-64,
x86-64-v2, x86-64-v3 and x86-64-v4 are listed before i386 in the gcc
documentation, but this nevertheless gives a good explanation for the
overall ordering of the list.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

arch/Config.in.x86

@@ -22,6 +22,9 @@ config BR2_X86_CPU_HAS_AVX2
 config BR2_X86_CPU_HAS_AVX512
 	bool
 
+# This list of CPU architecture variant is (loosely) ordered according
+# to the gcc documentation at
+# https://gcc.gnu.org/onlinedocs/gcc-11.2.0/gcc/x86-Options.html
 choice
 	prompt "Target Architecture Variant"
 	default BR2_x86_i586 if BR2_i386