[ruby/zlib] In Zlib::GzipReader#eof? check if we're actually at eof

	Only consider it eof if we read ahead and something fills the buf.
	If not, we may only have empty blocks and the footer.

	Fixes https://github.com/ruby/zlib/pull/56

	https://github.com/ruby/zlib/commit/437bea8003

This reverts commit https://github.com/ruby/zlib/commit/9f3b9c470c05 because we will
fix the issue on alternative way.

https://github.com/ruby/zlib/commit/3dfe3f9110

This commit fixes the test failures on the zlib in Ubuntu jammy s390x.

According to the <https://packages.ubuntu.com/jammy-updates/zlib1g> -
`zlib_1.2.11.dfsg-2ubuntu9.2.debian.tar.xz`,
the zlib deb package is applying the patch 410.patch (madler/zlib#410), and
configured by `./configure --dfltcc` on Ubuntu jammy s390x. The `--dfltcc`
is to enable the deflate algorithm in hardware.

It produces a different (but still valid) compressed byte stream, and causes
the test failures in ruby/zlib. As a workaround, set the environment variable
`DFLTCC=0` disabling the implementation in zlib on s390x to the failing tests.

Note we need to test in a child Ruby process with `assert_separately` to test
on the `DFLTCC=0` set by the parent Ruby process.

https://github.com/ruby/zlib/commit/9f3b9c470c

* The latest release does not have this fix:
  https://github.com/oracle/truffleruby/commit/c77f8bb35db084c99d1f5b14748267866004222e

https://github.com/ruby/zlib/commit/8abc80b994

(https://github.com/ruby/zlib/pull/55)

zstream_discard_input was encoding and character-aware when given input is user-provided, so this discards `len` chars instead of `len` bytes.

Also Zlib.gunzip explains in its rdoc that it is equivalent with the following code, but this doesn't fail for UTF-8 String.

```ruby
string = %w[1f8b0800c28000000003cb48cdc9c9070086a6103605000000].pack("H*").force_encoding('UTF-8')
sio = StringIO.new(string)
p gz.read #=> "hello"
gz&.close
p Zlib.gunzip(string) #=> Zlib::DataError
```

Reported and discovered by eagletmt at https://twitter.com/eagletmt/status/1689692467929694209

https://github.com/ruby/zlib/commit/c5e58bc62a

https://github.com/ruby/zlib/commit/fc89ff9ef0

Upper bits affect the result of `crc32` in zlib 1.2.12.

https://github.com/ruby/zlib/commit/9ab6d04af1

https://github.com/ruby/zlib/commit/5f23cd3009