This is still a POC, and would be split into multiple commits if it
This adds build-id support in userland that defaults to off but can
be enabled with WITH_BUILD_ID.
This also adds a "gdb_index" linker feature that is enabled for lld
8.0+. (I know it caused lld to crash in 6. I haven't tested on 7,
but I did bootstrap head from older VMs that had 8.0.1. I've been using
it with 9.0.1 on stable/12 to build GDB binaries where it works well).
The kernel just enables it unconditionally. Userland uses a
WITH_GDB_INDEX that defaults to off. This requires several diffs to
disable it when used with -r since lld complains.