sim/ppc: Fix core_find_mapping diagnostics

Because "%p" is the pointer conversion specifier to print a pointer in an
implementation-defined manner, the result with format string containing
"0x%p" can be strange.  For instance, core_map_find_mapping prints error
containing "0x0x...." (processor is not NULL) or "0x(null)" (processor is
NULL) on glibc.

This commit replaces "0x%p" with "%p" to prevent unpredictable behavior.