Review SharedArrayBuffer after atomics are implemented

[ghost]

The RISC-V specific code in TF_BUILTIN(AtomicsExchange, SharedArrayBufferBuiltinsAssembler) should be reviewed once atomics are properly implemented.

#if V8_TARGET_ARCH_MIPS || V8_TARGET_ARCH_MIPS64 || V8_TARGET_ARCH_RISCV64
  USE(array_buffer);
  TNode<Number> index_number = ChangeUintPtrToTagged(index_word);
  Return(CallRuntime(Runtime::kAtomicsExchange, context, array, index_number,
                     value));
#else

[ghost]

See also TF_BUILTIN(AtomicsCompareExchange, SharedArrayBufferBuiltinsAssembler):

#if V8_TARGET_ARCH_MIPS || V8_TARGET_ARCH_MIPS64 || V8_TARGET_ARCH_PPC64 || \
    V8_TARGET_ARCH_PPC || V8_TARGET_ARCH_S390 || V8_TARGET_ARCH_S390X ||    \
    V8_TARGET_ARCH_RISCV64
  USE(array_buffer);
  TNode<Number> index_number = ChangeUintPtrToTagged(index_word);
  Return(CallRuntime(Runtime::kAtomicsCompareExchange, context, array,
                     index_number, old_value, new_value));
#else

[ghost]

And SharedArrayBufferBuiltinsAssembler::AtomicBinopBuiltinCommon:

#if V8_TARGET_ARCH_MIPS || V8_TARGET_ARCH_MIPS64 || V8_TARGET_ARCH_PPC64 || \
    V8_TARGET_ARCH_PPC || V8_TARGET_ARCH_S390 || V8_TARGET_ARCH_S390X ||    \
    V8_TARGET_ARCH_RISCV64
  USE(array_buffer);
  TNode<Number> index_number = ChangeUintPtrToTagged(index_word);
  Return(CallRuntime(runtime_function, context, array, index_number, value));
#else