cast the arg to md_assemble () to char *

cast the arg to md_assemble () to char *

For these targets its not clear how md_assemble can usefully be split up so
that part can take const char *.  There is also a fair number of targets that
need md_assemble () to take a char *, so we can't easily make the argument
const.  So since there isn't many callers it seems easiest to just add a couple
of casts.

gas/ChangeLog:

2016-03-31  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* config/tc-m32c.c (m32c_md_end): cast the argument to md_assemble to
	char *.
	(m32c_indirect_operand): Likewise.
	* config/tc-nds32.c (do_pseudo_b): Likewise.
	(do_pseudo_bal): Likewise.
	(do_pseudo_ls_bhw): Likewise.