Add a .file directive to generated .s files

Add a .file directive to generated .s files

When no .file directive is given, the toolchain records the filename
of the .o file, which is sometimes random, making generated objects
non-deterministic.

We use Location.input_name for adding .file directives to assembly
files. Note: when the file is preprocessed, this reference holds the
name of the temporary file. Hence, files compiled with -pp are still
not deterministic.

Bug-Debian: https://bugs.debian.org/795784
Bug-Debian: https://bugs.debian.org/796336

Gbp-Pq: Name 0010-Add-a-.file-directive-to-generated-.s-files.patch