coff: only store PDB basename

[ypsvlq]

Moves towards reproducible builds on Windows and makes stack traces independent of cwd and zig-cache.
Helps #12729, but doesn't solve paths in the PDB.

[kubkon]

I might be misremembering things but didn't PDB/CodeView require abs paths?

[ypsvlq]

The information provided with /PDBALTPATH does not change the location or name of the actual .pdb file; it changes the information that the linker writes in the binary file. This enables you to provide a path that is independent of the file structure of the build computer. Two common uses for this option are to provide a network path or a file that has no path information.

MS Docs

[andrewrk]

Previous review was invalid due to a mistake I made when reading the diff. Here's a new review instead.