Move backtrace-sys to #![no_std]

Author: alexcrichton

backtrace-sys/Cargo.toml

@@ -12,7 +12,7 @@ Bindings to the libbacktrace gcc library
 """
 
 [dependencies]
-libc = "0.2"
+libc = { version = "0.2", default-features = false }
 
 [build-dependencies]
 cc = "1.0"

backtrace-sys/src/lib.rs

@@ -1,4 +1,5 @@
 #![allow(bad_style)]
+#![no_std]
 
 extern crate libc;