[libunwind] Unwind information caching for x86-64

[Arun Sharma]

This patch series should apply cleanly to 0.98.5 + bk patch. After the 
patches, I get:

./Lperf-simple
unw_getcontext : cold avg=  398.159 nsec, warm avg=  350.475 nsec
unw_init_local : cold avg=  429.153 nsec, warm avg=  319.481 nsec
no cache        : unw_step : 1st= 3694.353 min= 3017.698 avg= 3265.029 nsec
global cache    : unw_step : 1st=  524.521 min=  199.817 avg=  215.836 nsec
per-thread cache: unw_step : 1st=  324.703 min=  199.817 avg=  225.965 nsec

        -Arun