Mini-OS: don't use objcopy --dump-section

Mini-OS: don't use objcopy --dump-section

The objcopy option "--dump-section" isn't supported in binutils before
version 2.25, which are still supported by the Xen build system.

Avoid that by using the "-O binary" format together with
"--only-section". This requires to set the "alloc" section flag.

Fixes: 33411a11f848 ("Mini-OS: hide all symbols not exported via EXPORT_SYMBOLS()")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>