[CMake] Introduce LLVM_TARGET_TRIPLE_ENV as an option to override LLVM_DEFAULT_TARGET_TRIPLE at runtime.

No behavior is changed if LLVM_TARGET_TRIPLE_ENV is blank or undefined.

If LLVM_TARGET_TRIPLE_ENV is "TEST_TARGET_TRIPLE" and $TEST_TARGET_TRIPLE is not blank,
llvm::sys::getDefaultTargetTriple() returns $TEST_TARGET_TRIPLE.
Lit resets config.target_triple and config.environment[LLVM_TARGET_TRIPLE_ENV] to change the default target.

Without changing LLVM_DEFAULT_TARGET_TRIPLE nor rebuilding, lit can be run;

  TEST_TARGET_TRIPLE=i686-pc-win32 bin/llvm-lit -sv path/to/test/
  TEST_TARGET_TRIPLE=i686-pc-win32 ninja check-clang-tools

Differential Revision: https://reviews.llvm.org/D33662

llvm-svn: 305632