gas/arc: Allow greater range of characters into flag names

gas/arc: Allow greater range of characters into flag names

An upcoming commit will add a new arc instruction flag that uses
characters that have never appeared in an arc instruction flag before.
Currently the assembler is very conservative about which characters can
or cannot appear in an instruction flag.

This commit relaxes these constraints a little.  After this commit all
alpha-numeric characters are now allowed into instruction flags.  This
complete set is not required for the upcoming change, however, having
this slightly larger set does not impact the assemblers ability to
correctly parse input, but does make it easier to add new flag to the
instruction table.

gas/ChangeLog:

	* config/tc-arc.c (tokenize_flags): Allow greater range of
	characters into flag names.