[libc] mbrtowc implementation (#144760)

implemented the internal and public mbrtowc as well as tests for the
public function.

---------

Co-authored-by: Sriya Pratipati <sriyap@google.com>

libc/config/linux/x86_64/entrypoints.txt

@@ -1244,6 +1244,9 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.sys.socket.recv
     libc.src.sys.socket.recvfrom
     libc.src.sys.socket.recvmsg
+
+    # wchar.h entrypoints
+    libc.src.wchar.mbrtowc
   )
 endif()