Elfhack is used to optimize ELF shared binaries on Linux/UNIX systems.
It decreases libxul.so size up to 9Mb (10%) and speedups startup time.

The current code is still oriented on binaries produced by traditional gcc compiler (and bfd linker).
To support clang compiler and lld linker, as well as possible future needs for building with lto, elfhack should be updated.

The final code of ESR68 seems satisfy all possible needs for years to come.

Note, build/unix/elfhack/* is an isolated thing that does not correlate with anything else (ie. no prerequisites in general).
All the commits below can be quiet applied cleanly step by step.

First 21 commits.

Note: part1 is applied a bit shifted (since some more recent commit is already in the code).
Both part4 and part17 is related to changes in moz.build files. The corresponding commits affect other parts as well (outsude elfhack), hence applied partial.

At this stage the code is just before "the global reformatting"...

Last two little commits:

22: bug #1542958 part1: 04eccd9f23a1
23: bug #1542958 part2: 09240f7397c1

Should be a bit rebased due to reformatted code. The common preliminary patch attached.

Will be in 2.53.4. No need to keep a separate bug around because only backports without additional code.
In unofficial 2.53.5 since yesterday. 1457482-2Only-elfhack.62a1.patch to 1606739-elfhack-74a1.patch