ARM: vmlinux.lds: move discarded sections to beginning

Rather than scattering the discarded sections throughout the linker
file, move them to the start.

Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Tested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>