armv8: prevent using THUMB

The predominantely 32-bit ARM targets try to compile the SPL in Thumb
mode to reduce code size.
The 64-bit AArch64 instruction set does not know an alternative, concise
encoding, so the Thumb build option should only be set for 32-bit
targets.
Likewise -marm machine options are only valid for ARMv7 targets.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>