Add write function for sstatus register

asm.S

@@ -49,7 +49,7 @@ REG_READ(sie, 0x104)
 REG_SET_CLEAR(sie, 0x104)
 REG_READ(sip, 0x144)
 REG_READ_WRITE(sscratch, 0x140)
-REG_READ(sstatus, 0x100)
+REG_READ_WRITE(sstatus, 0x100)
 REG_SET_CLEAR(sstatus, 0x100)
 REG_READ(stval, 0x143)
 REG_READ_WRITE(stvec, 0x105)

src/register/sstatus.rs

@@ -105,6 +105,7 @@ impl Sstatus {
 }
 
 read_csr_as!(Sstatus, 0x100, __read_sstatus);
+write_csr!(0x100, __write_sstatus);
 set!(0x100, __set_sstatus);
 clear!(0x100, __clear_sstatus);