vmlinux.lds.h: Add .gnu.version* to COMMON_DISCARDS

For vmlinux linking, no architecture uses the .gnu.version* sections,
so remove it via the COMMON_DISCARDS macro in preparation for adding
--orphan-handling=warn more widely. This is a work-around for what
appears to be a bug[1] in ld.bfd which warns for this synthetic section
even when none is found in input objects, and even when no section is
emitted for an output object[2].

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=26153
[2] https://lore.kernel.org/lkml/202006221524.CEB86E036B@keescook/

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Fangrui Song <maskray@google.com>
Cc: linux-arch@vger.kernel.org
Link: https://lore.kernel.org/r/20200821194310.3089815-3-keescook@chromium.org