[BE] Delete torch._dl extension (#84361)
And lots of complexity around the availability of RTLD_GLOBAL flags in `os` module
As this flag is always present since Python-3.3, see https://docs.python.org/3/library/os.html#os.RTLD_GLOBAL
Fixes https://github.com/pytorch/pytorch/issues/84351
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84361
Approved by: https://github.com/kit1980
