kernel: tickless: Rename _Swap to allow creation of macro

Future tickless kernel patches would be inserting some
code before call to Swap. To enable this it will create
a mcro named as the current _Swap which would call first
the tickless kernel code and then call the real __swap()

Jira: ZEP-339
Change-Id: Id778bfcee4f88982c958fcf22d7f04deb4bd572f
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>