Fix Windows/MSVC build issues (#1498)

Fix two issues of building WAMR on Windows:
- The build_llvm.py script calls itself, spawning instances faster than they expire,
   which makes Python3 eat up the entire RAM in a pretty short time.
- The MSVC compiler doesn't support preprocessor statements inside macro expressions.
  Two places inside bh_assert() were found.

wamr-compiler/build_llvm.py

@@ -11,4 +11,4 @@ import sys
 script = (
     pathlib.Path(__file__).parent.joinpath("../build-scripts/build_llvm.py").resolve()
 )
-subprocess.check_call([sys.executable, script.name])
+subprocess.check_call([sys.executable, script])