qemu64 CPU model is incorrect

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1860575
Reported by 'The Lemon Man' on 2020-01-22 :

At the moment the "qemu64" CPU is defined as follows:

```
        .vendor = CPUID_VENDOR_AMD,
        .family = 6,
        .model = 6,
        .stepping = 3,
```

According to Wikipedia [1] this means the CPU is defined as part of the
K7 family while the AMD64 ISA was only introduced with the K8 series!

This causes some software such as LLVM to notice the problem (32-bit cpu
with 64-bit capability reported in the cpuid flag) and produce various
error messages.

The simple solution would be to upgrade this definition to use the
Sledgehammer
family (15) instead.

[1] https://en.wikipedia.org/wiki/List_of_AMD_CPU_microarchitectures