rts: fix an unaligned load in nonmoving gc

This patch fixes an unaligned load in nonmoving gc by ensuring the
closure address is properly untagged first before attempting to
prefetch its header. The unaligned load is reported by
UndefinedBehaviorSanitizer:

```
rts/sm/NonMovingMark.c:921:9: runtime error: member access within misaligned address 0x0042005f3a71 for type 'StgClosure' (aka 'struct StgClosure_'), which requires 8 byte alignment
0x0042005f3a71: note: pointer points here
 00 00 00  98 43 13 8e 12 7f 00 00  50 3c 5f 00 42 00 00 00  58 17 b7 92 12 7f 00 00  89 cb 5e 00 42
              ^
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior rts/sm/NonMovingMark.c:921:9
```

This issue had previously gone unnoticed since it didn't really harm
runtime correctness, the invalid header address directly loaded from a
tagged pointer is only used as prefetch address and will not cause
segfaults. However, it still should be corrected because the prefetch
would be rendered useless by this issue, and untagging only involves a
single bitwise operation without memory access so it's cheap enough to
add.