Fix LTO Detection in LLVM Intrinsic Build

[nlordell]

Fixes #33

This PR fixes LTO detection in the LLVM intrinsic build script. It turns out that around Rust 1.55, cargo stopped propagating RUSTFLAGS environment variable to the build script invocation (and even actively removing it). It is replaced with CARGO_ENCODED_RUSTFLAGS which is meant to more accurately set the actual rustc flags that are being used.

Turning this on yields an immediate improvement in benchmarks:

U256::add               time:   [15.726 ns 15.764 ns 15.808 ns]
                        change: [-34.507% -34.304% -34.099%] (p = 0.00 < 0.05)
                        Performance has improved.