gdb/csky-tdep.c: fix -Wunused-but-set-variable error

Fix these, seen when building with clang 14:

      CXX    csky-tdep.o
    /home/simark/src/binutils-gdb/gdb/csky-tdep.c:332:7: error: variable 'need_dummy_stack' set but not used [-Werror,-Wunused-but-set-variable]
      int need_dummy_stack = 0;
          ^
    /home/simark/src/binutils-gdb/gdb/csky-tdep.c:805:12: error: variable 'offset' set but not used [-Werror,-Wunused-but-set-variable]
                  int offset = 0;
                      ^

Change-Id: I6703bcb50e83c50083f716f4084ef6aa30d659c4