xtensa: switch from text-section-literals to auto-litpools

Now that both binutils and gcc support auto-litpools use that option
instead of text-section-literals to be able to compile huge functions.

Fixes:
  http://autobuild.buildroot.net/results/dd384fe0ef02a4205bea66a4a16ca2062afe53b4/
  http://autobuild.buildroot.net/results/87dd357a4b883ea3cd75546b3d63c4c28245beee/
  http://autobuild.buildroot.net/results/b5bca00dec1ecb118c7fb9c10dee74c94809c831/


and many others.

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