chrono: improvements to IFUNC and native timers

  - check for GCC >= 4.6 (not only GLIBC >= 2.11.2) before using IFUNC
  - implement native IFUNC-based timer
  - implement fall-back mechanism when IFUNC is not available
  - clean up native TSC timers
Automatically ported from CMSSW_7_2_X #5309