Move LTO into its own "release-lto" profile

[ulyssa]

The AUR iamb-git package seems to be broken right now due to some kind of weirdness with the LTO. A user reported in #iamb-users:0x.badd.cafe that they get the following:

          /usr/bin/ld: /home/rek2/.cache/paru/clone/iamb-git/src/iamb/target/release/deps/iamb-dee5b80dcda0a0a7: hidden symbol `GFp_ia32cap_P' isn't defined
          /usr/bin/ld: final link failed: bad value
          collect2: error: ld returned 1 exit status
          
  = note: some `extern` functions couldn't be found; some native libraries may need to be installed or have their path specified
  = note: use the `-l` flag to specify native libraries to link
  = note: use the `cargo:rustc-link-lib` directive to specify the native libraries to link with Cargo (see https://doc.rust-lang.org/cargo/reference/build-scripts.html#cargorustc-link-libkindname)

error: could not compile `iamb` (bin "iamb") due to 1 previous error
==> ERROR: A failure occurred in build().
    Aborting...
error: failed to build 'iamb-git-0.1.e98d58a-1': 
error: packages failed to build: iamb-git-0.1.e98d58a-1

This seems to be the same issue seen in briansmith/ring#1444. It sounds like certain environments result in a mix of GCC and Clang/LLVM being used while building resulting, causing the issue. I'm going to move LTO into its own non-default profile so that people won't hit this. For folks who want smaller binaries (LTO shaves off ~10mb, so I plan to continue using it for the binaries on the releases page), you can build with:

cargo build --profile release-lto