share/mk: Substitute reproducible prefixes in dwarf info

[markj]

Summary

Ensure that releases built from different source paths are reproducible.
Currently they are not since src and obj paths appear in DWARF info.
There are a few uses of FILE in the src tree as well, they are
handled by this change as well.

Use /usr/src and /usr/obj as the canonical src and obj paths when
WITH_REPRODUCIBLE_BUILD is defined. Define a DEBUG_PREFIX variable
which is used to ask the compiler to substitute paths in output files
using -ffile-prefix-map.

Compat builds (i.e., lib32) complicate things, since they override
OBJROOT to point to obj-${compat} but still refer to headers from the
native objroot (e.g., llvm headers). I have a hack for this case, but
suggestions for better solutions are welcome.

[markj]

Does anyone have any comments on the patch? I'd like to commit it as part of a larger project to improve src build reproducibility.

[emaste]

Same comment as in D50953 -- I think this is directionally the right thing but think it's unfortunate that we have four instances of similar logic. (Of course we have other build bits that are also repeated in this same set of files, this is not an entirely new complaint.)

[markj]

In D50951#1182198, @emaste wrote:

Same comment as in D50953 -- I think this is directionally the right thing but think it's unfortunate that we have four instances of similar logic. (Of course we have other build bits that are also repeated in this same set of files, this is not an entirely new complaint.)

I tried to minimize the harm by first deduplicating related bits into bsd.debug.mk: D51805. If that patch is ok, I will rebase this one on top of that.

[markj]

Rebase on top of bsd.debug.mk to eliminate the duplication