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PPC: `spr_write_xer` doesn't set flag bits in `cpu_xer`
Description of problem:
`spr_write_xer()` does not set the `ca`, `ov`, `so`, `ca32`, `ov32` etc. flag bits in the `cpu_xer` variable.
In fact it copies all bits from the source `GPR` and _excludes_ each flag bit.
This is not a problem for execution since `spr_read_xer()` gets the flag bits from `cpu_ca/ov/so...` and not from `cpu_xer`.
Nonetheless it is problem for tools which trace the execution in QEMU (e.g. https://github.com/BinaryAnalysisPlatform/qemu).
A fix would be to remove the `~` in https://gitlab.com/qemu-project/qemu/-/blob/master/target/ppc/translate.c#L481
Steps to reproduce:
Haven't found out yet how to debug QEMU so the TCGv values can be investigated. But in general one need to:
- Execute a binary which executes something like:
```
r4 = 0xffffffffffffffff
mtxer r4
```
and check the `cpu_xer` value after the `xer` write.
Checking the debug logs (`in_asm,cpu`) doesn't work, since the `xer` value in the logs is not taken directly from `cpu_xer`.
Additional information:
Code ref: https://gitlab.com/qemu-project/qemu/-/blob/master/target/ppc/translate.c#L480-L483
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