Debug info improvements:

- Create only one compile unit descriptor. We do not need more than one.
- Set compile unit directory to the current working directory.
- Do not create lexical block descriptors. As far as I know, the SSA
  package does not preserve this information.
- Do not split file paths. It is simpler to store the path in the "file"
  field and leave the "directory" field blank. It is also more
  correct, as the "directory" field is intended to be used for C/C++
  header search paths (see DWARF 4, section 6.2.4).

With this, we pass the log package's test suite.
Implement -fdebug-prefix-map flag
Emit correct debug info for functions with code from multiple source files

Such as the package init function.
Merge pull request #196 from pcc/libgo-p5

Debug info improvements