stand: Add libbacktrace to toolbox of boot loader

[imp]

Summary

libbacktrace is a combination of libunwind and libexecinfo from
userland, configured for the boot loader. At the present time, the
symbol translation has been disabled from libexecinfo. This will produce
numeric backtraces, which can be helpful in producing automatic
backtraces for exceptional events (like null pointer dereference).

Doesn't work on powerpc, so not enabled on powerpc*. It wasn't obvious
why the same thing that worked elsewhere failed here, so punt until
someone with more powerpc fu can figure it out.

Sponsored by: Netflix

[imp]

use alloca instead of heap alloc
Fix typos

[imp]

share/mk/src.opts.mk

135

likely need to add the docs to this too...

stand/libbacktrace/unwind-hacks.h

37

I know this is evil.

[imp]

add docs

[imp]

fprintf / flush were functions in early versions of this code, but switched to
defines since it was easier. Remove the no longer needed begin/end decls.

[jrtc27]

stand/libbacktrace/Makefile

67

That's not true, they get passed in different registers in some ABIs (amd64 uses XMM registers, aarch64 uses D registers). Though like the in-kernel printf floating-point formats aren't even supported in the implementation so whatever register it uses doesn't actually matter, it'll just stop processing formats and print the rest of the string verbatim. So it doesn't hit UB, but it also doesn't work as intended.

[imp]

Oh, and we don't need the assembler guard anymore since we're not declaring
C functions.

[jrtc27]

share/mk/src.opts.mk

322

How come? The userspace versions work just fine...

stand/libbacktrace/Makefile

72

This seems odd, why condition it?

[imp]

Improve comments in the crazy hack we do to get libunwind compliling.

[imp]

fix comment about libexecinfo. we don't use hack-unwind.h for libexecinfo.

[arichardson]

stand/libbacktrace/unwind-hacks.h

39

This sounds like something that should be fixed in libunwind with something like _LIBUNWIND_HAS_NO_FLOAT or inferred from soft float compiler flags. But I guess it's okay as an ugly workaround.