Prepare 9.1 Release
x86: lavr: use the x86inc.asm automatic stack alignment in mixing functions

CC:libav-stable@libav.org
(cherry picked from commit 95d01c3f1c33c079db04e24133ba4a6b00d62d4a)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>
update Changelog