exec: only build read_code when needed

Only build read_code when binary formats that use it are built into the
kernel.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Link: http://lkml.kernel.org/r/20200515143646.3857579-26-hch@lst.de


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>