Suppress optionrom build on Solaris x86

To avoid the build failing with:

gcc -Wall -Wstrict-prototypes -Werror -fomit-frame-pointer -fno-
builtin -I/export/home/andreas/QEMU/qemu   -Wall -Wstrict-prototypes -
Werror -fomit-frame-pointer -fno-builtin -I/export/home/andreas/QEMU/
qemu   -c -o multiboot.o multiboot.S
/var/tmp//ccd3aWyk.s: Assembler messages:
/var/tmp//ccd3aWyk.s:15: Error: value of 512 too large for field of 1
bytes at 0000000000000002
gmake[1]: *** [multiboot.o] Error 1

disable recursion into pc-bios/optionrom, as done for Darwin already.

Signed-off-by: Andreas Faerber <andreas.faerber@web.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>