[amoxor.w] rd gets the AMO result instead of the original memory value

[kkapapu]

Describe the bug

amoxor.w sp, a5, (t6)

• Sail / spec: rd = original memory value (sign-extended) 0xffffffff899fbaae
• DUT: rd = XOR result 0xffffffffa461e4eb

Expected behavior

RISC-V Unprivileged Spec, A extension: an AMO atomically loads from the address in rs1, places that original value into rd, applies the binary operator, and stores the result back to memory.

• Expected: sp = 0xffffffff899fbaae
• Actual: sp = 0xffffffffa461e4eb

The AMO FSM in UnpipelinedLSU.scala (s_amo_l / s_amo_a / s_amo_s) is unchanged on current master.

Environment

NutShell: 041f694