[llvm] Don't require third-party components when cross-compiling

It is possible to build LLVM with just the "llvm" and "cmake" components
checked out. This requires disabling the `LLVM_INCLUDE_BENCHMARKS` and
`LLVM_INCLUDE_TESTS` options. These options are not passed through to
the native build when cross-compiling though, so the build will break if
the "third-party" component is missing. We don't need the benchmarks or
tests for the native build, so disable these unconditionally. This fixes
cross-compiling on Gentoo, where only the required components are
checked out.

Differential Revision: https://reviews.llvm.org/D141196