PowerPC64 ELFv2 ABI 6/6: Bump ld.so soname version number

To avoid having a ELFv2 binary accidentally picking up an old ABI ld.so,
this patch bumps the soname to ld64.so.2.

In theory (or for testing purposes) this will also allow co-installing
ld.so versions for both ABIs on the same system.  Note that the kernel
will already be able to load executables of both ABIs.  However, there
is currently no plan to use that theoretical possibility in a any
supported distribution environment ...

Note that in order to check which ABI to use, we need to invoke the
compiler to check the _CALL_ELF macro; this is done in a new configure
check in sysdeps/unix/sysv/linux/powerpc/powerpc64/configure.ac,
replacing the hard-coded value of default-abi in the Makefile.