No system loads skiboot as an ELF, so the segment information is not
used.

Avoids these warnings with binutils as of 2.39:

 powerpc64-linux-gnu-ld: warning: skiboot.tmp.elf has a LOAD segment with RWX permissions
 powerpc64-linux-gnu-ld: warning: skiboot.elf has a LOAD segment with RWX permissions

Signed-off-by: Joel Stanley <joel@jms.id.au>
[arbab: Use try-ldflag for builders with pre-2.39 binutils]
Signed-off-by: Reza Arbab <arbab@linux.ibm.com>