devel/qt6-base: Add KDE-recommended upstream patches [1]

While here, add devel/libb2 as a LIB_DEPENDS, which is used automatically
if installed. Upstream prefers this implementation of BLAKE2. [2]

[2] https://code.qt.io/cgit/qt/qtbase.git/commit/?h=6.8&id=c19bafc60c1c2e9a73cb3d212293462a1be4eb4

Reported by: https://mail.kde.org/pipermail/distributions/2025-February/001559.html [1]
MFH: 2025Q1

(cherry picked from commit 641b06801d022e865192b72e085fbc027673f837)