std: Use backtrace-sys from crates.io

This commit switches the standard library to using the `backtrace-sys`
crate from crates.io instead of duplicating the logic here in the Rust
repositor with the `backtrace-sys`'s crate's logic.

Eventually this will hopefully be a good step towards using the
`backtrace` crate directly from crates.io itself, but we're not quite
there yet! Hopefully this is a small incremental first step we can take.

src/libstd/sys_common/backtrace.rs

@@ -53,6 +53,14 @@ const MAX_NB_FRAMES: usize = 100;
 pub fn print(w: &mut dyn Write, format: PrintFormat) -> io::Result<()> {
     static LOCK: Mutex = Mutex::new();
 
+    // There are issues currently linking libbacktrace into tests, and in
+    // general during libstd's own unit tests we're not testing this path. In
+    // test mode immediately return here to optimize away any references to the
+    // libbacktrace symbols
+    if cfg!(test) {
+        return Ok(())
+    }
+
     // Use a lock to prevent mixed output in multithreading context.
     // Some platforms also requires it, like `SymFromAddr` on Windows.
     unsafe {