Log Message:
x11/qt5-qtbase: build fix for older gcc -march=native

See https://bugreports.qt.io/browse/QTBUG-71564 . This replaces -march=core-avx2
with -mavx2 for the respective bits of the code, to avoid interaction
with global -march=naitive in not freshest GCC. Otherwise, the build would
fail in that setup.