Improve CMake output of host and target triple

Summary:
The previous output was confusing as it would output "Taget triple:
x86_64-unknown-linux-gnu" even when LLVM_HOST_TRIPLE or
LLVM_DEFAULT_TARGET_TRIPLE were set on the CMake command line

Patch by: Alex Richardson!

Reviewers: beanz

Subscribers: Eugene.Zelenko

Differential Revision: https://reviews.llvm.org/D17067

llvm-svn: 282516
2 files changed