[UPSTREAM][MIPS] Fix ll/sc after 7dd547e

After 7dd547e the env->llval value is
loaded as an unsigned value (instead of sign-extended as before).
Therefore, the CMPXCHG in gen_st_cond() in translate.c fails if the sign
bit is set in the loaded value.

I discovered this issue because FreeBSD MIPS64 was looping forever in an
atomic helper function when trying to start /sbin/init.

Feel free to apply this patch upstream, as I cannot submit the patch
myself due to the complicated contribution process.

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>

target/mips/op_helper.c

@@ -422,7 +422,7 @@ target_ulong helper_##name(CPUMIPSState *env, target_ulong arg, int mem_idx)  \
     env->llval = cpu_##insn##_mmuidx_ra(env, arg, mem_idx, GETPC());          \
     return env->llval;                                                        \
 }
-HELPER_LD_ATOMIC(ll, ldl, 0x3)
+HELPER_LD_ATOMIC(ll, ldsw, 0x3)
 #ifdef TARGET_MIPS64
 HELPER_LD_ATOMIC(lld, ldq, 0x7)
 #endif