ICU-22450 Disable the type_info workaround on newer versions of libstdc++

[glandium]

The workaround for http://llvm.org/bugs/show_bug.cgi?id=13364 is related to an ancient version of libstdc++ (4.6). On the flip side, with more recent versions of GCC, the workaround can cause problems under some conditions.

The _GLIBCXX_RELEASE macro was introduced in libstdc++ 7, checking for it not being defined to enable the workaround makes it disabled with libstdc++ >= 7, which don't need the workaround in the first place.

Checklist

• [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22450
• [x] Required: The PR title must be prefixed with a JIRA Issue number.
• [x] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [X] Required: Each commit message must be prefixed with a JIRA Issue number.
• [ ] Issue accepted (done by Technical Committee after discussion)
• [ ] Tests included, if applicable
• [ ] API docs and/or User Guide docs changed or added, if applicable

[CLAassistant]

All committers have signed the CLA.

[yumaoka]

@glandium Can you sign CLA? Otherwise, we cannot look at your changes.

[glandium]

I'm sorting this out through my employer. (an email has been sent to member-services at unicode.org about this. My work email is different than the one in the current commit, though, so I'll update that.

[jira-pull-request-webhook[bot]]

Hooray! The files in the branch are the same across the force-push. 😃

~ Your Friendly Jira-GitHub PR Checker Bot

[glandium]

The CLA should be sorted out.

[glandium]

Can this be merged?