emulators/qemu: fix build on powerpc64 elfv2

Clang doesn't like -mbig:
cc -mbig -c -o spapr-rtas.o spapr-rtas.S
cc: error: unknown argument: '-mbig'

This file doesn't seem to be built on amd64, so removing -mbig doesn't cause a problem there. I also successfully built on powerpc64 elfv1 and it also builds.

PR: 243188
Approved by: bofh (maintainer timeout)