plat/kvm: Select `HAVE_IBMPC` only on x86_64

plat/kvm: Select `HAVE_IBMPC` only on x86_64

Without this patch, asking `kraft` to build for QEMU on arm64 with its
default configuration gives a series of warnings such as:

```
WARNING: unmet direct dependencies detected for HAVE_IBMPC_VGA
  Depends on [n]: ARCH_X86_64 [=n]
  Selected by [y]:
  - HAVE_IBMPC [=y]
```

and `HAVE_IBMPC` is set to yes in the generated configuration.

Signed-off-by: Samuel Hym <samuel@tarides.com>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
Approved-by: Michalis Pappas <michalis@unikraft.io>
GitHub-Closes: #1565