add per-thread cpu clock for Mach / OS X
implement a generic serialising RDTSC interface

choose the best alternative (RDTSCP, LFENCE+RDTSC, MFENCE+RDTSC) at run time,
via the IFUNC dispath method and an ad-hoc resolver
split support for the "native duration" type into the native/ directory
reorganise the directory stucture
add a clock based on the generic serialising RDTSC interface
reorganise the directory stucture (again)
work around conflict between IFUNC and LTO
cleanup
implement the TSC decetion logic in the IFUNC resolver