vmlinux.lds.h: Create COMMON_DISCARDS

Collect the common DISCARD sections for architectures that need more
specialized discard control than what the standard DISCARDS section
provides.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: linux-arch@vger.kernel.org
Link: https://lore.kernel.org/r/20200821194310.3089815-2-keescook@chromium.org