When debugging i#6499 we noticed that drcachesim was producing 0 byte
read/write records for some SVE load/store instructions:

```
 ifetch       4 byte(s) @ 0x0000000000405b3c a54a4681   ld1w   (%x20,%x10,lsl #2) %p1/z -> %z1.s
 read         0 byte(s) @ 0x0000000000954e80 by PC 0x0000000000405b3c
 read         0 byte(s) @ 0x0000000000954e84 by PC 0x0000000000405b3c
 read         0 byte(s) @ 0x0000000000954e88 by PC 0x0000000000405b3c
 read         0 byte(s) @ 0x0000000000954e8c by PC 0x0000000000405b3c
 read         0 byte(s) @ 0x0000000000954e90 by PC 0x0000000000405b3c
 read         0 byte(s) @ 0x0000000000954e94 by PC 0x0000000000405b3c
 read         0 byte(s) @ 0x0000000000954e98 by PC 0x0000000000405b3c
 read         0 byte(s) @ 0x0000000000954e9c by PC 0x0000000000405b3c
 ifetch       4 byte(s) @ 0x0000000000405b4
```

This turned out to be due to drdecode being linked into drcachesim
twice: once into the drcachesim executable, once into libdynamorio.
drdecode uses a global variable to store the SVE vector length to use
when decoding so we end up with two copies of that variable and only
one was being initialized.
To fix this properly we would need to refactor the libraries so that
there is only one copy of the sve_veclen global variable, or change the
way that the decoder gets the vector length so its no longer stored in
a global variable. In the mean time we have a workaround which
makes sure both copies of the variable get initialized and drcachesim
produces correct results.

With that workaround in place however, the results were still wrong.
For expanded scatter/gather instructions when you are using an offline
trace, raw2trace doesn't have access to the load/store instructions
from the expansion, only the original app scatter/gather instruction.
It has to create the read/write records using only information from the
original scatter/gather instruction and it uses the size of the memory
operand to determine the size of each read/write. This works for x86
because the x86 IR uses the per-element data size as for the memory
operand of scatter/gather instructions. This doesn't work for AArch64
because the AArch64 codec uses the maximum data transferred
(per-element data size * number of elements) like other SIMD load/store
instructions.

We plan to make the AArch64 IR consistent with x86 by changing it to
use the same convention as x86 for scatter/gather instructions but in
the mean time we can work around the inconsistency by fixing the size
in raw2trace based on the instruction's opcode.

Issues: #6499, #5365

Change-Id: Idd84efa6be879af2b12f849c2143212cb48dacc1

Change-Id: I82d48ab7fc69e485a9f92dc2d85254723bf2f51a