[cmake] Use CMAKE_TRY_COMPILE_PLATFORM_VARIABLES

Now that our minimum required CMake version is past 3.6, we can use
CMAKE_TRY_COMPILE_PLATFORM_VARIABLES instead of relying on environment
variable trickery. The two aren't entirely equivalent because
CMAKE_TRY_COMPILE_PLATFORM_VARIABLES is only used for try_compiles of
source files and not whole projects, but I ran LLVM configures before
and after this change and the generated CMakeCache.txt files were
identical, so this should be NFC for us.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D121035