Fix powerpc issue with the linker from Fedora 32

The linker from Fedora 32 complains:

powerpc64-linux-gnu-ld: powerpc/selftest.elf: error: PHDR segment not
 covered by LOAD segment

Let's introduce some fake PHDRs to the linker script to get this
working again.

Message-Id: <20200514192626.9950-7-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>