cpu: Remove CPU_COMMON

This macro is now always empty, so remove it.  This leaves the
entire contents of CPUArchState under the control of the guest
architecture.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

target/riscv/cpu.h

@@ -171,9 +171,6 @@ struct CPURISCVState {
 
     float_status fp_status;
 
-    /* QEMU */
-    CPU_COMMON
-
     /* Fields from here on are preserved across CPU reset. */
     QEMUTimer *timer; /* Internal timer */
 };