ICU-22916 Remove use of makedep from MVS build, update doc

[alexgubanow]

Removed use of makdep, since OpenXL do support -MMD and -MT. I have copy pasted rules from darwin, as it is also clang.
Jira ticket https://unicode-org.atlassian.net/browse/ICU-22916

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22916
• 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) @markusicu please review/accept
• Tests included, if applicable - build runs faster now :)
• API docs and/or User Guide docs changed or added, if applicable - doc updated, removed mention of makedep requirement.