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

This reverts commit 55c83179.
Now that binutils bug with --gc-section is fixed this workaroud is no
longer needed.

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