Fix gcc/printf/ISO C++ warning

Remove the use of the 't' length modifier to avoid a gcc warning. Based
on usage, 32 bits of precision is good enough for printing a stack
offset for a stack trace.

't' length modifier isn't in C++03 but it *is* in C++11. Added a FIXME
to reintroduce once LLVM makes the switch to C++11.

Reviewer: gribozavr