rustbuild: make backtraces (RUST_BACKTRACE) optional

but keep them enabled by default to maintain the status quo.

When disabled shaves ~56KB off every x86_64-unknown-linux-gnu
binary.

To disable backtraces you have to use a config.toml (see
src/bootstrap/config.toml.example for details) when building rustc/std:

$ python bootstrap.py --config=config.toml

src/libstd/sys/unix/mod.rs

@@ -30,6 +30,7 @@ use libc;
 pub mod weak;
 
 pub mod android;
+#[cfg(feature = "backtrace")]
 pub mod backtrace;
 pub mod condvar;
 pub mod ext;