When building for arm32 with GNU binutils 2.39, the linker outputs
warnings when generating some TEE core binaries (all_obj.o, init.o,
unpaged.o and tee.elf) as well as ldelf.elf:

 arm-poky-linux-gnueabi-ld.bfd: warning: atomic_a32.o: missing .note.GNU-stack section implies executable stack
 arm-poky-linux-gnueabi-ld.bfd: NOTE: This behaviour is deprecated and will be removed in a future version of the linker

The permissions used when mapping the TEE core stacks do not depend on
any metadata found in the ELF file. Similarly when the TEE core loads
ldelf it already creates a non-executable stack regardless of ELF
information. Therefore we can safely ignore the warnings. This is done
by adding the '--no-warn-execstack' option.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>