make microblaze build with -Wwrite-strings

make microblaze build with -Wwrite-strings

frag_var () assigns its last argument to frag::fr_opcode, and it turns out
some targets modify the string that points to.  However it appears niether the
generic code or the microblaze code modifies what fr_opcode points to, so this
code should be safe.  So we unfortunately need to cast to char * when passing
an argument to frag_var () but otherwise microblaze can itself point to these
strings with const char *.

gas/ChangeLog:

2016-03-24  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* config/tc-microblaze.c (md_assemble): Cast opc to char * when calling
	frag_var ().