CMake: Support for cross-compiling. For now, requires a previously

CMake: Support for cross-compiling. For now, requires a previously
built native tblgen which is passed to cmake in the variable
LLVM_TABLEGEN.

See

http://www.cmake.org/Wiki/CmakeMingw

for a quick example on how to cross-compile with CMake.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@58939 91177308-0d34-0410-b5e6-96231b3b80d8