Some mips64 binaries have ELFCLASS32

`irsb = pyvex.IRSB('\xdf\xbf\x00\x18\xdf\xbc\x00\x10\xdf\xb1\x00\x08\xdf\xb0\x00\x00', 0x5193c, archinfo.ArchMIPS32('Iend_BE'), opt_level=0)`

gives:

```
Traceback (most recent call last):
File "", line 1, in
File "/home/user/workspace/angr-dev/pyvex/pyvex/block.py", line 64, in __init__
lift(self, data, max_bytes, max_inst, bytes_offset, opt_level, traceflags)
File "/home/user/workspace/angr-dev/pyvex/pyvex/lift/__init__.py", line 100, in lift
raise PyVEXError('\n\n'.join(errors))
pyvex.errors.PyVEXError:
vex: priv/guest_mips_toIR.c:1219 (putIReg): Assertion `typeOfIRExpr(irsb->tyenv, e) == ty' failed.
```

The bytes should be:
```
0x5193c ld $ra, 0x20+var_8($sp)
0x51940 ld $gp, 0x20+var_10($sp)
0x51944 ld $s1, 0x20+var_18($sp)
0x51948 ld $s0, 0x20+var_20($sp)
```
(ODA confirms)