[libc] Forward LLVM_LIBC options when using a runtimes build

The `LLVM_ENABLE_RUNTIMES' mode is commonly used to build runtimes that
depend on an up-to-date version of clang. Currently, `libc` uses some
internal variables that are not forwarded when building in this mode.
This patch forwards the relevent arguments beginning with `LLVM_LIBC` to
the build when built this way.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D137977