stand: Add eh symbols to linker scripts

[imp]

Summary

Add eh_* symbols to the arm, arm64, i386, amd64 and riscv linker
scripts. They are needed to create tracebacks with libunwind. Without
addition arguments, these use no additional space in the binaries (flags
that I have no plans to add to even the debug versions since we do
exceptions in the boot loaders). Generally placed in the same location
as other places in the tree, except for where we have a pehdr section
(in which case I place them abitrarily last in the text section). These
are needed by libunwind, but defining this way will effectively disable
them. Libunwind has no easy way to disable them via a #define like so
many other aspects of its behavior.

Didn't do powerpc, nor uboot since those loaders are legacy, seen to
have unclear layout requirements and I have no way to test the results.

Sponsored by: Netflix

Test Plan

build before / after notice the 'size' size is the same.

[dim]

You might want to clarify that this is specifically about boot loader linker scripts?

For the rest I'm fine with it.

[andrew]

Do we need to add .eh_frame_hdr to the objcopy command in stand/efi/loader/Makefile?

[arichardson]

I'm surprised these are needed, ld.lld should automatically add the symbols. Also can the _hdr symbols be placed inside the section? It should be fine for them to be be non-zero if non-existent since end will be equal to start.

[imp]

In D37864#860541, @arichardson wrote:

I'm surprised these are needed, ld.lld should automatically add the symbols. Also can the _hdr symbols be placed inside the section? It should be fine for them to be be non-zero if non-existent since end will be equal to start.

I've not tried it without the eh_frame_hdr* symbols, to be honest. I hit the eh_frame_start/end being missing, found a comment and added what the comment said :)