[arm] Inhibit linker warnings about an implied executable stack

[arm] Inhibit linker warnings about an implied executable stack

Some versions of the 32-bit ARM linker seem to treat the absence of a
.note.GNU-stack section as implying an executable stack, and will
print a warning that this is deprecated behaviour.

Silence the warning by adding a .note.GNU-stack section to each
assembly file and retaining the sections in the Linux linker script.

Signed-off-by: Michael Brown <mcb30@ipxe.org>