kmod: disable --gc-sections ld option when building for xtensa

This is a workaround for the following ld issue exhibited by --gc-sections
option, until a proper fix for binutils is ready:

ld: BFD (GNU Binutils) 2.22 internal error, aborting at elf32-xtensa.c line 3374 in elf_xtensa_finish_dynamic_sections

Fixes:

  http://autobuild.buildroot.org/results/97d/97d4c96d6f6cdc1ed4007456f4ab70be9dfa41b5/



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