add symlinks for $TRIPLE-clang-cpp

[h-vetinari]

In conda-forge/openjdk-feedstock#167, the build checks presence and functionality of the preprocessor. For linux, this is handled as CPP=${CXX_FOR_BUILD//+/p}, where CXX_FOR_BUILD generally uses the $TRIPLE-<name> style.

We're similarly setting

clang-compiler-activation-feedstock/recipe/build.sh

Lines 29 to 30 in 04e1556

	CC_FOR_BUILD=${CBUILD}-clang
	CXX_FOR_BUILD=${CBUILD}-clang++

here, but contrary to gcc_impl_linux-<arch>, we don't have a $TRIPLE-preprocessor for clang. Since the symlink structure is not the same for gcc & clang anyway (c.f. #118), add a symlink here for $TRIPLE-clang-cpp, like we do for $TRIPLE-clang already.

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.