devel/qt6-base: fix build on armv6/armv7
The port bundles assembly code from the pixman library.
On armv6/armv7, the GNU assembler is required to assemble it.
Pass appropriate flags to do so.
PR: 266829
(cherry picked from commit 76fa8695d9c814e0d5e9b8e28bcf6e897422b435)