Use consistent prefix map when building deterministic system libraries (#23222)

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
#23195 (comment)
while discussins `__FILE__` problem in #23195.

This does not change any code size tests because no code size tests
happens to contain `__FILE__` at the moment. (One will be added in
#22994)

Author: aheejin

test/test_other.py

@@ -10409,7 +10409,7 @@ def test_dwarf_system_lib(self):
         [LLVM_DWARFDUMP, libc, '-debug-info', '-debug-line', '--recurse-depth=0'],
         stdout=PIPE).stdout
     # Check that the embedded location of the source file is correct.
-    self.assertIn('DW_AT_name\t("system/lib/emmalloc.c")', dwdump)
+    self.assertIn('DW_AT_name\t("/emsdk/emscripten/system/lib/emmalloc.c")', dwdump)
     self.assertIn('DW_AT_comp_dir\t("/emsdk/emscripten")', dwdump)
 
   @parameterized({

tools/system_libs.py

@@ -42,6 +42,10 @@
 # link time.
 USE_NINJA = int(os.environ.get('EMCC_USE_NINJA', '0'))
 
+# A (fake) deterministic emscripten path to use in __FILE__ macro and debug info
+# to produce reproducible builds across platforms.
+DETERMINISITIC_PREFIX = '/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}={DETERMINISITIC_PREFIX}',
+                 f'-ffile-prefix-map={relative_source_dir}={DETERMINISITIC_PREFIX}',
+                 f'-fdebug-compilation-dir={DETERMINISITIC_PREFIX}']
     asflags = get_base_cflags(preprocess=False)
     input_files = self.get_files()
     ninja_file = os.path.join(build_dir, 'build.ninja')
@@ -496,9 +500,9 @@ def build_objects(self, build_dir):
       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}={DETERMINISITIC_PREFIX}']
+      cflags += [f'-ffile-prefix-map={source_dir}={DETERMINISITIC_PREFIX}',
+                 f'-fdebug-compilation-dir={DETERMINISITIC_PREFIX}']
     case_insensitive = is_case_insensitive(build_dir)
     for src in self.get_files():
       ext = shared.suffix(src)