nios2: Don't disable relaxation with --gdwarf-N

GCC 11 passes --gdwarf-5 to assembler to enable DWARF5 debug info.  Don't
disable relaxation when --gdwarf-N is specified.  The assembler generated
debug information will treat the sequence of the relaxed instructions as
a single instruction.

	PR gas/27243
	* config/tc-nios2.c (md_begin): Don't disable relaxation with
	--gdwarf-N.
	* testsuite/gas/nios2/relax.d: New file.
	* testsuite/gas/nios2/relax.s: Likewise.