[RISCV] relaxDwarfCallFrameFragment: remove unneeded relocations for relaxation

If `evaluateAsAbsolute(Value, Layout.getAssembler())` returns true, we
know the address delta is a constant and can suppress relocations
(usually SET6/SUB6).

While here, replace one evaluateKnownAbsolute call (subtle for Mach-O
workarounds; avoid if possible) with evaluateAsAbsolute.