[Bazel] Remove dependency on system python on Windows too (#1681)

This is a followup of #1676.

Just a heads up with those changes (and the ones from #1676), things
will not work anymore for projects that don't use an hermetic python
toolchain because the interpreter will not be found.

Using the system python is not a great idea anyway, but I just wanted to
mention that because that forced our hand in adopting the hermetic
toolchain (we were planing on doing it at a later date), and I don't
know if this had been considered in #1676.

Author: stevenlr

bazel/emscripten_toolchain/emar.bat

@@ -2,4 +2,4 @@
 
 call %~dp0\env.bat
 
-py -3 %EMSCRIPTEN%\emar.py %*
+"%EMSDK_PYTHON%" %EMSCRIPTEN%\emar.py %*

bazel/emscripten_toolchain/emcc.bat

@@ -2,4 +2,4 @@
 
 call %~dp0\env.bat
 
-py -3 %EMSCRIPTEN%\emcc.py %*
+"%EMSDK_PYTHON%" %EMSCRIPTEN%\emcc.py %*

bazel/emscripten_toolchain/emcc_link.bat

@@ -2,4 +2,4 @@
 
 call %~dp0\env.bat
 
-py -3 %~dp0\link_wrapper.py %*
+"%EMSDK_PYTHON%" %~dp0\link_wrapper.py %*