www/qt5-webengine: fix build on armv7

The build system adds the bogus option --target=arm-linux-gnueabihf to
the build when building on arm, causing all sorts of issues. Patch that
out, then add some polyfill for API differences between ARM Linux and
ARM FreeBSD.

Approved by: portmgr (build fix blanket)
MFH: 2025Q2