cl: eliminate uses of the CL_LINK_GCC_PLUGIN macro

... and inline its only remaining instantiation
cl: rename CL_BUILD_GCC_PLUGIN -> CL_BUILD_COMPILER_PLUGIN
cl,sl: fix warnings reported by Clang
cl/llvm: add support for Clang/LLVM as a front-end
cl/llvm: re-use __builtin_alloca_with_align()

... instead of creating a new built-in function
cl/llvm: remove clname.cc and clplug.exports

... which are no longer useful
cl: rename CL_BINOP_BAD -> CL_BINOP_UNKNOWN

... in order to make the identifier more predictable based on the
already existing CL_TYPE_UNKNOWN identifier.
switch-host-clang.sh: do not use CLANG_HOST as CC/CXX

We need to strictly distinguish build/host/target architectures,
compilers, and compiler flags.
switch-host-clang.sh: build fwnull and vra, too
run-llvm-tests.sh: eliminate a run-time error

... when invoked with no args:

./run-llvm-tests.sh: line 38: [: -e: binary operator expected