cad/ghdl: Update 5.1.1 => 6.0.0

[alven]

Summary

• Add WWW for main site
• Fix SYNTH logic to optionally disable it as by default synth is enabled https://github.com/ghdl/ghdl/blob/e589c698c351369ac5bcfe7abe1f1152ac5d4727/configure#L52
• Simplify stripping logic in post-install phase

Test Plan

For every {150amd64, 144amd64}:

• Successful poudriere testport
• Successful run-testing

[yuri]

Thanks for the patch!
Approved.