Allow host triple to be correctly overridden in CMake builds

The intended semantics mirror autoconf, where the user is able to
specify a host triple, but if it's left to the build system then
"config.guess" is invoked for the default.

This also renames the LLVM_HOSTTRIPLE define to LLVM_HOST_TRIPLE to
fit in with the style of the surrounding defines.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181112 91177308-0d34-0410-b5e6-96231b3b80d8