Turn embind into MTLibrary

Newly added _emval_coro_reject() function calls JS Promise reject
functor at the end of call hierarchy. Since the call crosses C++ JS
barrier, the Signature::get_method_caller() function is called, which
has thread_local variable, adding thread local storage, and therefore
the requirement for the embind to be compiled with same threadness as
the main program.

Author: davits

tools/system_libs.py

@@ -1925,7 +1925,7 @@ class libwebgpu_cpp(MTLibrary):
   src_files = ['webgpu_cpp.cpp']
 
 
-class libembind(Library):
+class libembind(MTLibrary):
   name = 'libembind'
   never_force = True