lang/rust: Remove the LLVM_PORT option

FTR, this option allowed to used LLVM from ports instead of building the
bundled copy.

The problem is that this combination isn't really tested upstream. This
led to regressions which are difficult to diagnose. For instance, in
Rust 1.21.0, the bundled LLVM provided a new API to query the features
supported by the target arch. The equivalent code inside Rust was
removed to use that new API. Unfortunately, building Rust 1.21.0+ with a
copy of LLVM not providing this API didn't failed but instead made that
list of CPU features empty. This resulted in the following obscure build
failure in Firefox:

error[E0432]: unresolved import `x86::sse2`

To avoid future pain for both end users and maintainers, we decided to
remove that option. Yes, it will increase the (already long) time to
build Rust, but it should save time wasted on debugging what is not
really supported anyway.

PR: 223342, 223300
Reported by: Many users