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.

src/Cargo.lock

@@ -100,7 +100,7 @@ name = "backtrace"
 version = "0.3.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "backtrace-sys 0.1.23 (registry+https://github.com/rust-lang/crates.io-index)",
+ "backtrace-sys 0.1.24 (registry+https://github.com/rust-lang/crates.io-index)",
  "cfg-if 0.1.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "libc 0.2.42 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc-demangle 0.1.8 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -109,7 +109,7 @@ dependencies = [
 
 [[package]]
 name = "backtrace-sys"
-version = "0.1.23"
+version = "0.1.24"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "cc 1.0.18 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -3218,7 +3218,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 "checksum assert_cli 0.6.2 (registry+https://github.com/rust-lang/crates.io-index)" = "98589b0e465a6c510d95fceebd365bb79bedece7f6e18a480897f2015f85ec51"
 "checksum atty 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)" = "9a7d5b8723950951411ee34d271d99dddcc2035a16ab25310ea2c8cfd4369652"
 "checksum backtrace 0.3.9 (registry+https://github.com/rust-lang/crates.io-index)" = "89a47830402e9981c5c41223151efcced65a0510c13097c769cede7efb34782a"
-"checksum backtrace-sys 0.1.23 (registry+https://github.com/rust-lang/crates.io-index)" = "bff67d0c06556c0b8e6b5f090f0eac52d950d9dfd1d35ba04e4ca3543eaf6a7e"
+"checksum backtrace-sys 0.1.24 (registry+https://github.com/rust-lang/crates.io-index)" = "c66d56ac8dabd07f6aacdaf633f4b8262f5b3601a810a0dcddffd5c22c69daa0"
 "checksum bitflags 0.9.1 (registry+https://github.com/rust-lang/crates.io-index)" = "4efd02e230a02e18f92fc2735f44597385ed02ad8f831e7c1c1156ee5e1ab3a5"
 "checksum bitflags 1.0.3 (registry+https://github.com/rust-lang/crates.io-index)" = "d0c54bb8f454c567f21197eefcdbf5679d0bd99f2ddbe52e84c77061952e6789"
 "checksum bufstream 0.1.3 (registry+https://github.com/rust-lang/crates.io-index)" = "f2f382711e76b9de6c744cc00d0497baba02fb00a787f088c879f01d09468e32"