Adding a warning tells the user they can migrate (which upgrades x86 meterp to x64) e.g:

```
meterpreter > sysinfo
Computer        : WIN7-X64
OS              : Windows 7 (Build 7601, Service Pack 1).
Architecture    : x64 (Current Process is WOW64)
System Language : en_GB
Meterpreter     : x86/win32
meterpreter > migrate 1976
[*] Migrating from 2444 to 1976...
[*] Migration completed successfully.
meterpreter > sysinfo
Computer        : WIN7-X64
OS              : Windows 7 (Build 7601, Service Pack 1).
Architecture    : x64
System Language : en_GB
Meterpreter     : x64/win64
```