Use /emsdk/emscripten for relative path substitution

When `deterministic_paths` is set, we are currently using
`-ffile-prefix-map` to produce the same path in data and debug info. In
the case of absolute paths, their emscripten path is replaced with a
fake path `/emsdk/emscripten`, and in the case of relative paths, all
path relative to the emscripten directory is removed, so
`../../system/lib/somefile.c` becomes `system/lib/somefiles.c`.
https://github.com/emscripten-core/emscripten/blob/f66b5d706e174d9e5cc6122c06ea29dcd2735cd0/tools/system_libs.py#L472-L477
https://github.com/emscripten-core/emscripten/blob/f66b5d706e174d9e5cc6122c06ea29dcd2735cd0/tools/system_libs.py#L495-L501

So this does not make relative paths and absolute paths the same, which
can lead to different builds depending on whether the command line uses
absolute paths vs. relative ones. Currently we use relative paths when
`EMCC_BATCH_BUILD` is set. And Ninja builds cannot use relative paths.

This is also what was suggested in
emscripten-core#23195 (comment)
while discussins `__FILE__` problem in #23915.

test/other/codesize/test_codesize_cxx_except.size

@@ -1 +1 @@
-171008
+171030

test/other/codesize/test_codesize_cxx_except_wasm.size

@@ -1 +1 @@
-142223
+142234

test/other/codesize/test_codesize_cxx_except_wasm_exnref.size

@@ -1 +1 @@
-144810
+144821

test/other/codesize/test_codesize_cxx_mangle.size

@@ -1 +1 @@
-232517
+232548

tools/system_libs.py

@@ -42,6 +42,10 @@
 # link time.
 USE_NINJA = int(os.environ.get('EMCC_USE_NINJA', '0'))
 
+# A fake emscripten path to use in __FILE__ macro and debug info to produce
+# reproducible builds across platforms.
+FAKE_EMSCRIPTEN_PATH = '/emsdk/emscripten'
+
 
 def files_in_path(path, filenames):
   srcdir = utils.path_from_root(path)
@@ -472,9 +476,9 @@ def generate_ninja(self, build_dir, libname):
     if self.deterministic_paths:
       source_dir = utils.path_from_root()
       relative_source_dir = os.path.relpath(source_dir, build_dir)
-      cflags += [f'-ffile-prefix-map={source_dir}=/emsdk/emscripten',
-                 f'-ffile-prefix-map={relative_source_dir}/=',
-                 '-fdebug-compilation-dir=/emsdk/emscripten']
+      cflags += [f'-ffile-prefix-map={source_dir}={FAKE_EMSCRIPTEN_PATH}',
+                 f'-ffile-prefix-map={relative_source_dir}={FAKE_EMSCRIPTEN_PATH}',
+                 '-fdebug-compilation-dir={FAKE_EMSCRIPTEN_PATH}']
     asflags = get_base_cflags(preprocess=False)
     input_files = self.get_files()
     ninja_file = os.path.join(build_dir, 'build.ninja')
@@ -492,13 +496,14 @@ def build_objects(self, build_dir):
     commands = []
     objects = set()
     cflags = self.get_cflags()
-    if self.deterministic_paths:
+    if True:
+    #if self.deterministic_paths:
       source_dir = utils.path_from_root()
       if batch_inputs:
         relative_source_dir = os.path.relpath(source_dir, build_dir)
-        cflags += [f'-ffile-prefix-map={relative_source_dir}/=']
-      cflags += [f'-ffile-prefix-map={source_dir}=/emsdk/emscripten',
-                 '-fdebug-compilation-dir=/emsdk/emscripten']
+        cflags += [f'-ffile-prefix-map={relative_source_dir}={FAKE_EMSCRIPTEN_PATH}']
+      cflags += [f'-ffile-prefix-map={source_dir}={FAKE_EMSCRIPTEN_PATH}',
+                 '-fdebug-compilation-dir={FAKE_EMSCRIPTEN_PATH}']
     case_insensitive = is_case_insensitive(build_dir)
     for src in self.get_files():
       ext = shared.suffix(src)