powerpc: Fix ld.so address determination for PCREL mode (bug 31640)

This seems to have stopped working with some GCC 14 versions,
which clobber r2.  With other compilers, the kernel-provided
r2 value is still available at this point.

Reviewed-by: Peter Bergner <bergner@linux.ibm.com>
(cherry picked from commit 14e56bd4)