Fix CMake LLVM_TARGETS_TO_BUILD "Native" option to work with JIT

LLVM_TARGETS_TO_BUILD accepts both "host" or "Native" for auto-selecting
the target from the environment. However the way "Native" was plumbed
would lead to the JIT environment being disabled. This patch is making
"Native" works just as "host".

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D95837