Makefile: rename LLVM_TRIPLE to LLVM_TARGET

"Triple" is too generic, and it's actually not a triple, but a
quadruple.  "Target" appears to be a better name.  LLVM/clang
documentation calls it "target triple", and the clang option is called
"-target", not "-triple".
Makefile: always use LLVM_TARGET if present

Prepare to use the variable for other targets using clang.
Makefile: move LLVM target definitions to LLVM_TARGET