Cleanup implementation of target builtins code.

- Renamed stdlib-sse.ll to builtins-sse.ll (etc.) in an attempt to better indicate
the fact that the stuff in those files has a role beyond implementing stuff for
the standard library.
- Moved declarations of the various __pseudo_* functions from being done with LLVM
API calls in builtins.cpp to just straight up declarations in LLVM assembly
language in builtins.m4.  (Much less code to do it this way, and more clear what's
going on.)

stmt.cpp

@@ -1442,7 +1442,7 @@ lProcessPrintArg(Expr *expr, FunctionEmitContext *ctx, std::string &argTypes) {
 
 
 /* PrintStmt works closely with the __do_print() function implemented in
-   the stdlib-c.c file.  In particular, the EmitCode() method here needs to
+   the builtins-c.c file.  In particular, the EmitCode() method here needs to
    take the arguments passed to it from ispc and generate a valid call to
    __do_print() with the information that __do_print() then needs to do the
    actual printing work at runtime.