ATLEDGE-75 - Time: micros()
ATLEDGE-75 - Time: micros()

* Re-write micros() function in assembly for a 15-20% performance
  increase. It should take less than a microsecond now.

Signed-off-by: Bogdan Pricop <5a89b24d475f828b4ad2063082e1fda772239aff@emutex.com>

yashaswini-hanji/corelibs-arduino101,bigdinotech/corelibs-arduino101,SidLeung/corelibs-arduino101,eriknyquist/corelibs-arduino101,sgbihu/corelibs-arduino101,bigdinotech/corelibs-arduino101,01org/corelibs-arduino101,sgbihu/corelibs-arduino101,01org/corelibs-arduino101,01org/corelibs-arduino101,facchinm/corelibs-arduino101,sandeepmistry/corelibs-arduino101,linrjing/corelibs-arduino101,eriknyquist/corelibs-arduino101,yashaswini-hanji/corelibs-arduino101,SidLeung/corelibs-arduino101,yashaswini-hanji/corelibs-arduino101,sandeepmistry/corelibs-arduino101,facchinm/corelibs-arduino101,sgbihu/corelibs-arduino101,linrjing/corelibs-arduino101,sandeepmistry/corelibs-arduino101,bigdinotech/corelibs-arduino101