RISC-V: tighten post-relocation-operator separator expectation

As per the spec merely a blank isn't okay as a separator, the operand
to the relocation function ought to be parenthesized. Enforcing this
then also eliminates an inconsistency in that

	lui	t0, %hi sym
	lui	t0, %hi 0x1000

were accepted, but

	lui	t0, %hi +sym
	lui	t0, %hi -0x1000

were not.