PR 34036 looping in symbol_equated_p chains

This patch adds a new function for safely traversing a chain of equated
symbols to find the defining symbol.

	PR 34036
	* symbols.c (symbol_equated_to): New function.
	* symbols.h (symbol_equated_to): Declare.
	* expr.c (resolve_register): Use symbol_equated_to.
	* config/tc-i386.c (parse_register): Likewise.
	* config/tc-v850.c (reg_name_search): Likewise.