Maybe drop TARGET_COMPILER_xlc

Legacy XL C/C++ does not support C++11 or C++14. Open XL C/C++, the modern replacement, is derived from Clang and for all intents and purposes can be treated as Clang in source. With that in mind, we should cleanup build files to treat XL C/C++ similar to Clang, when possible, and fold TARGET_COMPILER_xlc to TARGET_COMPILER_gcc just like we do for Clang. We should also drop any legacy XL C/C++ specific stuff that also has approaches supported by GCC/Clang.