lib/csu: Ensure that ELF notes are retained with -Wl,--gc-sections

[arichardson]

Summary

This requires a rather recent clang (https://reviews.llvm.org/D95730),
so I'm not sure if we can commit this as-is now.

Test Plan

previously building a binary with -static and -Wl,--gc-sections on
would not have the FreeBSD ABI note, and on some architectures (e.g. aarch64)
this would result in the kernel ELF loader rejecting the file with:
ELF binary type "0" not known.
This patch is based on CheriBSD (first patch submitted from my Morello board), but I will update to drop the non-upstream parts if this is considered the correct approach to work around this problem.

[jrtc27]

This was considered but not done as it requires the linker to support SHF_RETAIN. The best option is to move the note to just crti and have a dummy crti for CHERI downstream rather than omit it

[arichardson]

In D35534#805923, @jrtc27 wrote:

This was considered but not done as it requires the linker to support SHF_RETAIN. The best option is to move the note to just crti and have a dummy crti for CHERI downstream rather than omit it

Makes sense, it is I was concerned about. How about downstream though, we don't have the old linker constraint there.

[emaste]

I think binutils allows (or allowed) SHF_GNU_RETAIN only with ELFOSABI_GNU or ELFOSABI_FREEBSD, and if we have that then this change is less important.