Add support for DWARF in PECOFF as used by cygwin and mingw

For cygwin and mingw targets, build a dump_sys tool which can read DWARF symbols
contained in a PE/COFF executable.

I felt bad about making another copy of dump_symbols.cc, so this one has the
beginnings of being parameterized by an image file reader class, so we can apply
it to a pecoff-format image file reading class

Write pecoff format image file reading class which
- knows how to read PE32 and PE32+ image files
- makes PECOFF file id by fetching from a CV record or by hashing
- can read PECOFF exports as a fallback if there is no DWARF information