When cross-compiling, `cfg!(target_os)` and friends will be the
configuration for the host rather than the target, since the build
script runs on the host. Cargo provides the target's configuration via
environment variables, so use those.

If we fail because there seems to be something wrong with library names,
provide more output on what's expected and what we found. This should
make cross-compilation related errors easier to spot, including Windows
target-env mismatches as in #24 (which was user error but not easy to
determine).

Be smarter about target-env

See merge request taricorp/llvm-sys.rs!13

I'm not sure why I thought these didn't exist, since on examination
upstream it seems the AsmPrinter has been implemented from very early in
the RISC-V backend's life; assume the others were too.

orc2: Make struct fields public

See merge request taricorp/llvm-sys.rs!14