Documentation/changes: Raise minimum supported binutils version to 2.23

The currently minimum-supported binutils version 2.21 has the problem of
promoting symbols which are defined outside of a section into absolute.
According to Arvind:

binutils-2.21 and -2.22. An x86-64 defconfig will fail with
Invalid absolute R_X86_64_32S relocation: _etext
and after fixing that one, with
Invalid absolute R_X86_64_32S relocation: __end_of_kernel_reserve

Those two versions of binutils have a bug when it comes to handling
symbols defined outside of a section and binutils 2.23 has the proper
fix, see: https://sourceware.org/legacy-ml/binutils/2012-06/msg00155.html

Therefore, up to the fixed version directly, skipping the broken ones.

Currently shipping distros already have the fixed binutils version so
there should be no breakage resulting from this.

For more details about the whole thing, see the thread in Link.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200110202349.1881840-1-nivedita@alum.mit.edu
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>