x86/AT&T: make GOT-relative expressions work

x86/AT&T: make GOT-relative expressions work

The expressions used in intel-got{32,64}.s should equally work (or not) in
AT&T mode. Changing the Intel syntax parser such that O_symbol wouldn't
happen to be wrapped around such expressions breaks it there, too. It
really isn't correct to limit this to just O_symbol. Permitting O_add and
O_subtract as well requires taking care of the other operand as well then.
(Strictly speaking non-zero offsets aren't very useful here, but then at
least with an equate of 0 this ought to work. The non-zero offset in the
testcase helps demonstrate that this offset isn't lost.)