pc-bios/s390-ccw: Introduce `EXTRA_LDFLAGS`

pc-bios/s390-ccw: Introduce `EXTRA_LDFLAGS`

Some packaging tools want to override `LDFLAGS` when building QEMU, this will
result in a build error as most likely no `-nostdlib` flag is passed. Introduce
`EXTRA_LDFLAGS` so that the packager can override `LDFLAGS` without breaking the
build.

Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Message-ID: <20241001153618.17791-4-mhartmay@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
[thuth: Drop the hunk to netbook.mak which is not necessary anymore]
Signed-off-by: Thomas Huth <thuth@redhat.com>