extend char16_t workaround to cygwin less than 3.5

[jeremyd2019]

uchar.h was added in Cygwin version 3.5.0. Unfortunately, the last version of Cygwin that supported i686 was 3.3.6, and I think 3.4 was the last version that supported Windows 7, so it is still somewhat relevant to a subset of users.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-_____
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• 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.

[FrankYFTang]

we love to review your PR but cannot do so until you sign the CLA.

[cjchapman]

@jeremyd2019 Thank you for signing the CLA. Will you please create a JIRA ticket for this?

[jeremyd2019]

I don't want to create a new account with another service just to create a ticket.