LinuxKPI: add ktime_get_real_ts64()

[bz]

Summary

Add a wrapper for ktime_get_real_ts64() used by a wireless driver.

MFC after: 10 days

[emaste]

I'm not sure if this one needs to be the non-get version, could be getnanotime?

[bz]

In D40246#916350, @emaste wrote:

I'm not sure if this one needs to be the non-get version, could be getnanotime?

Neither am I. We do use nanotime() for ktime_get_real and ktime_get_real_seconds and then convert it.
My understanding was that our "get*" version was faster and less precise so I figured I'll follow the other "real" cases already in the file?

[emaste]

AFAICT "real" refers to it being time of day, rather than not-cached.

[bz]

Switch to get-version as pointed out by @emaste.