PR26448 UBSAN: symbols.c:1586 left shift of negative value

PR26448 UBSAN: symbols.c:1586 left shift of negative value

Besides avoiding the UB, this also makes right shifts inside
expression symbols unsigned, consistent with the way gas evaluates
expressions in source.

	PR 26448
	* symbols.c: Include limits.h.
	(resolve_symbol_value <O_left_shift, O_right_shift>): Do an
	unsigned shift.  Warn if shift count larger than valueT size.