2.2 build fails with GCC 4.1.2 on 32-bit architectures

InnoDB 5.6 cannot be built in the following configuration:

- gcc 4.1.2
- 32-bit architecture
- Performance Schema disabled at compile time

The build fails as follows:

---
08:05:26 [ 70%] Building CXX object storage/innobase/CMakeFiles/innobase.dir/srv/srv0mon.cc.o
08:05:27 /home/jenkins/workspace/percona-xtrabackup-2.2/BUILD_TYPE/release/Host/ubuntu-lucid-32bit/xtrabackuptarget/innodb55/storage/innobase/srv/srv0mon.cc:1215: error: ‘mysql_pfs_key_t’ does not name a type
08:05:27 make[2]: *** [storage/innobase/CMakeFiles/innobase.dir/srv/srv0mon.cc.o] Error 1
---

The reason is that HAVE_ATOMIC_BUILTINS_64 is not defined under the above conditions, but the code in srv0mon.cc assumes that PFS is always enabled at compile time.

Which is technically not an upstream bug, because the manual says that GCC 4.2.1 is the minimum supported version for 5.6. However, XtraBackup does need to build that code in the above configuration.