boost-context: Reproducibility: Set .file section for all *_elf_gas.S files

boost-context: Reproducibility: Set .file section for all *_elf_gas.S files

Add a .file directive explicitly for all *_elf_gas.S files to prevent the linker
adding a host build-system path as a FILE symbol to the object file.

This replaces the existing patch that added the .file directive to a small
subset of these files.

Upstream-Status: Submitted [https://github.com/boostorg/context/issues/91]
Signed-off-by: Douglas Royds <douglas.royds@taitradio.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>