[cmake] Fix host tools build in when LLVM_EXPERIMENTAL_TARGETS_TO_BUILD is set

r320413 triggered cmake configure failures when building with 
-DLLVM_OPTIMIZED_TABLEGEN=True and with LLVM_EXPERIMENTAL_TARGETS_TO_BUILD set 
(e.g. to RISCV). This is because that patch moved to passing through 
LLVM_TARGETS_TO_BUILD, and at that point LLVM_EXPERIMENTAL_TARGETS_TO_BUILD 
has been merged in to it. LLVM_EXPERIMENTAL_TARGETS_TO_BUILD must be also be 
passed through to avoid errors like below:

-- Constructing LLVMBuild project information
CMake Error at CMakeLists.txt:682 (message):
  The target `RISCV' does not exist.

      It should be one of


  AArch64;AMDGPU;ARM;BPF;Hexagon;Lanai;Mips;MSP430;NVPTX;PowerPC;Sparc;SystemZ;X86;XCore


-- Configuring incomplete, errors occurred!

See the thread
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20171211/509225.html 
for discussion of this fix.

llvm-svn: 320556