Fix a problem in tracing across fork on PPC64

Fix a problem in tracing across fork on PPC64

In order to avoid single-stepping through large portions of the
dynamic linker, ltrace remembers at which address the instruction that
resolved a PLT slot is.  It then puts a breakpoint to this address so
that it can fast-forward to that address next time it needs to catch a
PLT slot being resolved.

When a process is cloned, the pointer to this breakpoint is simply
copied over to the new process, instead of being looked up in the new
process structures.  This patches fixes this.