pseries: Update SLOF firmware image

This includes gcc8.1 fixes and the image is compiled using gcc 8.1 as well.

The full list of changes is:
  > Fix bad assembler statements for compiling with gcc 8.1 / as 2.30
  > libelf: Add REL32 to the list of ignored relocations

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

pc-bios/README

@@ -17,7 +17,7 @@
 - SLOF (Slimline Open Firmware) is a free IEEE 1275 Open Firmware
   implementation for certain IBM POWER hardware.  The sources are at
   https://github.com/aik/SLOF, and the image currently in qemu is
-  built from git tag qemu-slof-20180621.
+  built from git tag qemu-slof-20180702.
 
 - sgabios (the Serial Graphics Adapter option ROM) provides a means for
   legacy x86 software to communicate with an attached serial console as

SLOF @ 9b7ab2fa

-Subproject commit 7d37babcfa48a6eb08e726a8d13b745cb2eebe1c
+Subproject commit 9b7ab2fa020341dee8bf9df6c9cf40003e0136df