Only allocatable code sections are relaxable.

This change is not critical, but is I believe correct.  Only allocatable
code sections will be relaxed (seebfd/elf32-avr.c:elf32_avr_relax_section).
This will be more use later as it will reduce the number of property
records that are emitted in order to support .org and .align directives.

gas/ChangeLog:

	* config/tc-avr.c (relaxable_section): Only allocatable code
	sections are relaxable.