Don't try to statically link libstdc++ on FreeBSD

The code inside this conditional will not work on FreeBSD 10+ because
those versions use clang and libc++ rather than libstdc++.

Since FreeBSD comes with libc++ in the base, presumably all 10+ systems
will have it present.

Searching for libstdc++.a will not work if it is not present.  As a
result, this would previously have set `LLVM_STATIC_STDCPP=libstdc++.a`,
which isn't a valid path and caused problems later on when building
`librustc_llvm`.

This could possibly be updated in the future to look for `libc++.a` on
FreeBSD, by expanding the code inside the conditional.  In one attempt
to run this on x86_64-freebsd, I found that libc++ was not compiled with
PIC, so it failed anyway.

src/bootstrap/compile.rs

@@ -550,6 +550,7 @@ pub fn rustc_cargo(build: &Build,
     // Building with a static libstdc++ is only supported on linux right now,
     // not for MSVC or macOS
     if build.config.llvm_static_stdcpp &&
+       !target.contains("freebsd") &&
        !target.contains("windows") &&
        !target.contains("apple") {
         cargo.env("LLVM_STATIC_STDCPP",