civetweb: fix build for xtensa

The civetweb package bundled sqlite3 generates an object that is 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/d14/d142f3ce17ab22cc39f9117c114318c1b5cadfc5/

.

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