x86-64: fix suffix-less PUSH of symbol address

PR gas/30856

In 5cc00775 ("x86: further adjust extend-to-32bit-address
conditions") I neglected the case of PUSH, which is the only insn
allowing (proper) symbol addresses to be used as immediates (not
displacements, like CALL/JMP) in the absence of any register operands.
Since it defaults to 64-bit operand size, guessing an L suffix is wrong
there.