ARM: fix LTO build for some thumb-interwork cases

Fix LTO build for some thumb-interwork usecases (such as for
da850evm_defconfig), where inline assmebly such as
  mrc p15,0,r2,c1,c0,0
causes the compiler to fail during LTO linking with
  Error: selected processor does not support `mrc p15,0,r2,c1,c0,0'
         in Thumb mode

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>