[llvm] Include LLVM_ENABLE_RUNTIMES in the cross compilation invocation

Forward the LLVM_ENABLE_RUNTIMES cmake variable in the cross compilation
invocation.

Now that passing libcxx must be specified in LLVM_ENABLE_RUNTIMES
instead of LLVM_ENABLE_PROJECTS, the LLDB's sanity check for libcxx
trips up (in the cross compilation case) because the runtimes are not
forwarded. This patch fixes that.

Differential revision: https://reviews.llvm.org/D112855