php: fix build for xtensa

The php package generates a binary 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/a9a/a9a1063104402ec28e01560ec7c8f8a5b6d43dd5/

.

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