Makefile.in: add -mtext-section-literals to xtensa ABI

Collecting literals into separate section can be advantageous if that
section is placed into DTCM at link time. This is applicable for code
running on bare metal, but makes no sense under linux, where userspace
is isolated from the physical memory details. OTOH placing literals into
separate section breaks build of huge source files, because l32r
instruction can only access literals in 256 KBytes range.

Add -mtext-section-literals into xtensa ABI to fix build issues of
packages with huge sources.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>