gas various other const pointer changes

This removes a bunch of casts involving const pointers, in some cases
by making variables const pointers so a cast is not needed.  In a
couple of places the cast hid errors with "&array" written rather than
"array", see iq2000_macro_defs and s_pru_align.  tc-xgate.c cmp_opcode
is changed to be the standard qsort predicate to avoid a function
cast.