Process stack display (not currently used).
This is a set of publicly-available debugging aids which all do pretty well the same thing: display the contents of the stack, along with symbolic information, for a running process. There are versions for various object file formats (such as ELF and IEEE-695). Most of the programs are copyrighted by the Free Software Foundation and are marked as "part of GNU Binutils".
In other words, the pstack files are not really part of the MySQL library. They are merely useful when you re-program some MYSQL code and it crashes.