Make sure that compiler and linker don't optimize the section's contents
away by adding the global holding the data to "llvm.used". The volatile
load in the main shim is retained because "llvm.used", which translates
to SHF_GNU_RETAIN on ELF targets, requires a reasonably recent linker;
emitting the volatile load ensures compatibility with older linkers, at
least when libstd is used.
Pretty printers in dylib dependencies are now emitted by the main crate
instead of the dylib; apart from matching how rlibs are handled, this
approach has the advantage that `omit_gdb_pretty_printer_section` keeps
working with dylib dependencies.
Some tests fail on cross-compiled targets due to various linker problems
on cross-compiled target, and having test coverage for these against
cross-compiled targets is nice but not necessary.
I'm not entirely sure this is correct, but it doesn't feel
obviously-wrong so I figured I'd just start by sending a PR rather than
filing a bug and letting it linger.
@rustbot label llvm-main
