[OpenMP] Fixing llvm-omp-device-info compilation with runtimes

When using `-DLLVM_ENABLED_RUNTIMES` instead of `-DLLVM_ENABLED_PROJECTS`
the `llvm-omp-device-info` tool is not compiled or installed.
In general, no llvm tool would be build on runtimes, because the
-DLLVM_BUILD_TOOLS flag is removed by the way runtimes compilation calls
cmake again.

This patch is simple. Just forward the value of this flag to the
runtime cmake command.

I'm also removing an unnecessary comment in the compilation of the tool

Differential Revision: https://reviews.llvm.org/D107177

(cherry picked from commit 5424ceed)