6.8 prep glibc 2.32

Hi,

I'm unsure how to time this right, but from Ubuntu (which already has glibc 2.32) we know that we then need to adapt the XDR handling. I thought I propose it now for a discussion upfront and we can then rebase and apply whenever 2.32 hit Debian (is there an ETA anywhere?)

We first of all need rph.h back which will be covered with the tirpc build-dependency. And then we should be able to drop the revert of the patch. But there should be a test build that checks the .so files if xdr is properly mapped there.

Wrong:

eu-readelf -a /usr/lib/libvirt/libvirt_lxc | grep xdr_uint64 | grep GLOBAL
  104: 0000000000000000 0 FUNC GLOBAL DEFAULT UNDEF xdr_uint64_t@GLIBC_2.2.5 (4)

Right (against tirpc):

eu-readelf -a /usr/lib/libvirt/libvirt_lxc | grep xdr_uint64 | grep GLOBAL
   85: 0000000000000000 0 FUNC GLOBAL DEFAULT UNDEF xdr_uint64_t@TIRPC_0.3.0 (8)

P.S. I'll give this a try in the Ubuntu 21.04 builds for libvirt 6.8 right now.