avfilter: Add missing emms_c when needed

Arch specific calls should have an emms_c following to keep the cpu
state consistent.

Reported-By: wm4
CC: libav-stable@libav.org
matroskaenc: allow override of "writing application" tag

Signed-off-by: Tim Walker <tdskywalker@gmail.com>

CC: libav-stable@libav.org
(cherry picked from commit 0092c1dd8dac2d9e185b58503b447a0d3fb5230d)