cross-x86_64-linux-musl: install linux API headers before musl to DESTDIR.

... otherwise some musl provided headers (scsi/sg.h) might be removed
by this without any apparent reason.
cross-i686-linux-musl: install linux API headers before musl to DESTDIR.

... otherwise some musl provided headers (scsi/sg.h) might be removed
by this without any apparent reason.
cross-arm-linux-musl: install linux API headers before musl to DESTDIR.

... otherwise some musl provided headers (scsi/sg.h) might be removed
by this without any apparent reason.

And disable libssp as was done to other cross-*-musl toolchains.