Confirm offset in CallCFunctionHelper

[ghost]

This FIXME needs to be checked and fixed or removed. It seems correct, or else I would expect some tests to fail, but someone needs to go through and confirm.

    auipc(pc_scratch, 0);
    // FIXME(RISCV): Does this need an offset? It seems like this should be the
    // PC of the call, but MIPS does not seem to do that.

[luyahan]

In arm64 it's offset is also 0.

  Label get_pc;
  Bind(&get_pc);
  Adr(pc_scratch, &get_pc);

[luyahan]

see arm:

  // Save the frame pointer and PC so that the stack layout remains iterable,
  // even without an ExitFrame which normally exists between JS and C frames.

This should save current pc to stack. So we can delete fixme