Tweak the documentation of libiberty's xcrc32 function

Tweak the documentation of libiberty's xcrc32 function

libiberty/ChangeLog;

	* crc32.c: In the documentation, don't refer to GDB's
	now-nonexistent crc32 implementation.  In the table-generation
	program embedded within the documentation, change the type of
	the induction variables i and j from int to unsigned int, to
	avoid undefined behavior.

From-SVN: r231983