ARM: avoid marking decompressor .stack section as having contents

The .stack section doesn't contain any contents, and doesn't require
initialization either.  Rather than marking the output section with
'NOLOAD' but still having it exist in the object files, mark it with
%nobits which avoids the assembler marking the section with 'CONTENTS'.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>