ffmpeg: fix build for xtensa

The ffmpeg package generates libraries that are too large for the xtensa
default placement of literals in a dedicated section. Use
-mtext-section-literal to place literals in the text section.

Fixes
http://autobuild.buildroot.net/results/606/60670d35f16c3b8fe19debf7f8e40a046a579520/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>