kbuild: mkcompile_h: Include $LD version in /proc/version

When doing Clang builds of the kernel, it is possible to link with
either ld.bfd (binutils) or ld.lld (LLVM), but it is not possible to
discover this from a running kernel. Add the "$LD -v" output to
/proc/version.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Fangrui Song <maskray@google.com>
Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>