Add "generic" 4, 8, and 16-wide targets.

When used, these targets end up with calls to undefined functions for all
of the various special vector stuff ispc needs to compile ispc programs
(masked store, gather, min/max, sqrt, etc.).

These targets are not yet useful for anything, but are a step toward
having an option to C++ code with calls out to intrinsics.

Reorganized the directory structure a bit and put the LLVM bitcode used
to define target-specific stuff (as well as some generic built-ins stuff)
into a builtins/ directory.

Note that for building on Windows, it's now necessary to set a LLVM_VERSION
environment variable (with values like LLVM_2_9, LLVM_3_0, LLVM_3_1svn, etc.)

func.cpp

@@ -288,7 +288,10 @@ Function::emitCode(FunctionEmitContext *ctx, llvm::Function *function,
         bool checkMask = (type->isTask == true) || 
             ((function->hasFnAttr(llvm::Attribute::AlwaysInline) == false) &&
              costEstimate > CHECK_MASK_AT_FUNCTION_START_COST);
-        if (checkMask && g->opt.disableCoherentControlFlow == false) {
+        checkMask &= (g->target.maskingIsFree == false);
+        checkMask &= (g->opt.disableCoherentControlFlow == false);
+
+        if (checkMask) {
             llvm::Value *mask = ctx->GetFunctionMask();
             llvm::Value *allOn = ctx->All(mask);
             llvm::BasicBlock *bbAllOn = ctx->CreateBasicBlock("all_on");