std: Use target_pointer_width for BACKTRACE_ELF_SIZE

The former code used `target.contains("64")` to detect Elf64 targets,
but this is inaccurate in a few cases:

- `s390x-unknown-linux-gnu` is 64-bit
- `sparcv9-sun-solaris` is 64-bit
- `x86_64-unknown-linux-gnux32` is 32-bit

Instead the `std` build script can use `CARGO_CFG_TARGET_POINTER_WIDTH`
to reliably detect 64-bit targets for libbacktrace.

Also update to backtrace-sys 0.1.24 for rust-lang/backtrace-rs#122.

Author: cuviper

src/libstd/build.rs

@@ -104,7 +104,8 @@ fn build_libbacktrace(target: &str) -> Result<(), ()> {
     } else {
         build.file("../libbacktrace/elf.c");
 
-        if target.contains("64") {
+        let pointer_width = env::var("CARGO_CFG_TARGET_POINTER_WIDTH").unwrap();
+        if pointer_width == "64" {
             build.define("BACKTRACE_ELF_SIZE", "64");
         } else {
             build.define("BACKTRACE_ELF_SIZE", "32");