Remove use of llvm-mc (#18298)

llvm-mc is more an internal LLVM tool than a user-facing replacement for as.
Instead just use clang in the file packager (the one place we need to build assembly files).

Author: dschuff

tools/file_packager.py

@@ -345,9 +345,9 @@ def generate_object_file(data_files):
     target = 'wasm64-unknown-emscripten'
   else:
     target = 'wasm32-unknown-emscripten'
-  shared.check_call([shared.LLVM_MC,
-                     '-filetype=obj',
-                     '-triple=' + target,
+  shared.check_call([shared.EMCC,
+                     '-c',
+                     '--target=' + target,
                      '-o', options.obj_output,
                      asm_file])
 

tools/shared.py

@@ -747,7 +747,6 @@ def get_llvm_target():
 LLVM_RANLIB = build_llvm_tool_path(exe_suffix('llvm-ranlib'))
 LLVM_OPT = os.path.expanduser(build_llvm_tool_path(exe_suffix('opt')))
 LLVM_NM = os.path.expanduser(build_llvm_tool_path(exe_suffix('llvm-nm')))
-LLVM_MC = os.path.expanduser(build_llvm_tool_path(exe_suffix('llvm-mc')))
 LLVM_DWARFDUMP = os.path.expanduser(build_llvm_tool_path(exe_suffix('llvm-dwarfdump')))
 LLVM_OBJCOPY = os.path.expanduser(build_llvm_tool_path(exe_suffix('llvm-objcopy')))
 LLVM_STRIP = os.path.expanduser(build_llvm_tool_path(exe_suffix('llvm-strip')))