gas: equates of registers

There are two problems: symbol_equated_p() doesn't recognize equates of
registers, and S_CAN_BE_REDEFINED() goes by section rather than by
expression type. Both together undermine .eqv and .equiv clearly meaning
to guard the involved symbols against re-definition (both ways).

To compensate pseudo_set() now using O_symbol and S_CAN_BE_REDEFINED()
now checking for O_register,
- for targets creating register symbols through symbol_{new,create}() ->
  symbol_init() -> S_SET_VALUE() (alpha, arc, dlx, ia64, m68k, mips,
  mmix, tic4x, tic54x, plus anything using cgen or itbl-ops), have
  symbol_init() set their expressions to O_register,
- x86'es parse_register() also can't go by section anymore when
  trying to "look through" equates; probably symbol_equated_p() should
  have been used there from the beginning, if only that had worked for
  equates of registers,
- various targets need to "look through" equates when parsing insn
  operands (which also helps transitive forward equates); perhaps even
  more ought to, but many don't look to consider the possibility of
  register equates in the first place.

This was uncovered by code reported in PR gas/30274 (duplicating
PR gas/30272), except that there .eqv was used when really .equ was
meant. Therefore that bug report is addressed here only in so far as
gas wouldn't crash anymore; the code there still won't assemble
successfully, just that now the issues there are properly diagnosed.

gas/expr.h

@@ -190,5 +190,6 @@ extern symbolS *expr_build_dot (void);
 extern uint32_t generic_bignum_to_int32 (void);
 extern uint64_t generic_bignum_to_int64 (void);
 extern int resolve_expression (expressionS *);
+extern void resolve_register (expressionS *);
 
 extern bool literal_prefix_dollar_hex;