Depends on backtrace-sys even when rustc-dep-of-std is the only feature

[RalfJung]

When disabling all features except for rustc-dep-of-std, this crate should have minimal dependencies and, in particular, it would be great if it could build without a C toolchain for the target. That is needed to keep libstd (with no features) buildable without a C toolchain (modulo rust-lang/rust#56443).

However, currently, backtrace-sys is always built when building libstd, even with no features. The reason for this is probably that rustc-dep-of-std enables backtrace-sys/rustc-dep-of-std, which unfortunately implicitly enables backtrace-sys.

@alexcrichton suggests to work around this by making backtrace-sys with no features do basically nothing, and then enabling the "actually-build" feature only from backtrace's "libbacktrace" feature.