cleanup FLT_CHARS and EXP_CHARS

Providing declarations in tc.h points out that alpha wasn't properly marking
FLT_CHARS as const.  We can also get rid of the confusing redefinition of
X_CHARS to mmix_x_chars.  Finally we can get rid of some now redundant
declarations of these constants.

gas/ChangeLog:

2016-04-03  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* config/tc-alpha.c: Const qualify FLT_CHARS.
	* config/atof-ieee.c: Remove declarations of FLT_CHARS and EXP_CHARS.
	* config/tc-cris.h: Likewise.
	* expr.c: Likewise.
	* config/tc-mmix.c (md_atof): Adjust comment.
	* config/tc-mmix.h: Stop defining FLT_CHARS and EXP_CHARS as macros.
	* tc.h: Declare FLT_CHARS and EXP_CHARS.