Extract .debug files more intelligently, and grab available sources

[kergoth]

We should:

1. Determine which files from the external toolchain are ELF binaries (see package.bbclass for that)
2. Grab any path from .gnu_debuglink, use this to determine any .debug files to extract, in case the external toolchain has done a debug data split the way we do
3. Ensure we either install it to the expected path given the current debuglink, or alter it to match our configuration and expectations. Currently, for example, the Sourcery G++ toolchain does a debug split, but in a style that doesn't match any of the existing debug styles in OE
4. Go through the debug info, checking for existing source files and extracting those as well, and ideally moving them into /usr/src/debug and adjusting the paths in the debug info to match