ECOFF debug information (mdebug) included the concept of a PDR.  MIPS uses
these for unwinding through functions, among other things.  Right now we can
get them from mdebug or from code inspection.  But current GNU binutils
emits ".pdr" sections instead of using the old ".mdebug" style debug info.

This patch lets us read and handle the .pdr sections.  It's only for
32-bit, because for 64-bit gas currently emits only half the address.  That
I'll address later.  It changes a testsuite run from 5591 calls to
heuristic_proc_desc to 1787 calls.  Most or all of those are because we
build a heuristic procedure description if we find that we are still in the
prologue of a function.  The new ".pdr" method finds a PDR successfully for
nearly all calls to non_heuristic_proc_desc; the others are probably either a
lingering library on my system built by an older assembler, or signal/solib
trampolines.  Only 29 misses total.

There's an increase in memory usage per-objfile, but no leaks, and the
section is generally not large; for all of GDB it is only 78K.

Andrew, does this look OK to you?

-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer