make xtensa_section_rename () take const char *

make xtensa_section_rename () take const char *

Xtensa uses it in several macros passing it a literal string, so its convenient
for the argument type to be const char *.  However some of the code in
symbols.c seems to assume tc_canonicalize_symbol_name () will return a non
const pointer, and some other target's implementations even modify the
argument, so it seems best to return a char * which means casting away const on
the argument when we return it instead of another string.

gas/ChangeLog:

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

	* config/tc-xtensa.c (struct rename_section_struct): Make old_name
	const.
	(xtensa_section_rename): Make argument type const char *.
	* config/tc-xtensa.h (xtensa_section_rename): Adjust.