glibc-package.bbclass: Add supersecret -r switch to qemu userspace emulator,

glibc-package.bbclass: Add supersecret -r switch to qemu userspace emulator,
which allows to override kernel version report to guest apps (otherwise,
host kernel version is used, which causes "Kernel too old" errors in glibc
startup code in guess, if host kernel is older than 2.6.14-something).
Tested with qemu 0.8.2 (and yes, I couldn't find -r switch in qemu doc,
taken from a mail by Paul Brook, qemu co-maintainer).