Override DEBUG_BUILD for qemu as the qemu upstream states it
doesn't work without optimization [1] to fix below build failure
when debug build enabled.
| /usr/lib/gcc/x86_64-wrs-linux/10.1.0/../../../../x86_64-wrs-linux/bin/ld.bfd: /mnt/build/tmp/work/x86_64-linux/qemu-system-native/5.1.0-r0/qemu-5.1.0/fsdev/qemu-fsdev-throttle.c:25: undefined reference to `unknown_lock_type'
| /usr/lib/gcc/x86_64-wrs-linux/10.1.0/../../../../x86_64-wrs-linux/bin/ld.bfd: ../fsdev/qemu-fsdev-throttle.o: in function `fsdev_co_throttle_request':
| /mnt/build/tmp/work/x86_64-linux/qemu-system-native/5.1.0-r0/qemu-5.1.0/fsdev/qemu-fsdev-throttle.c:103: undefined reference to `unknown_lock_type'
| /usr/lib/gcc/x86_64-wrs-linux/10.1.0/../../../../x86_64-wrs-linux/bin/ld.bfd: ../fsdev/qemu-fsdev-throttle.o:/mnt/build/tmp/work/x86_64-linux/qemu-system-native/5.1.0-r0/qemu-5.1.0/fsdev/qemu-fsdev-throttle.c:103: more undefined references to `unknown_lock_type' follow
| collect2: error: ld returned 1 exit status
