community/qt6-qtwebengine: re-enable on loongarch64

• move source=

• remove duplicate options= and builddir=

• remove unused qt6-6.7.1 loongarch64 patches

• add qt6-6.8.0 loongarch64 patches, source:

  https://github.com/AOSC-Dev/chromium-loongarch64/tree/master/qt6-webengine

follow-up to !73248 (merged)