BaseTools/CommonLib: use explicit 64-bit type in Strtoi()
Don't use the native word size string to number parsing routines,
but instead, use the 64-bit one and cast to UINTN.
Currently, the only user is in Source/C/DevicePath/DevicePathFromText.c
which takes care to use Strtoi64 () unless it assumes the value fits
in 32-bit, so this change is a no-op even on 32-bit build hosts.