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

(cherry picked from commit 85ef7e6db838d5276da8c9d6b658f41cdda13e6c)