unwind: enable dwarf cache of libunwind

unwind: enable dwarf cache of libunwind

Here is the benchmark of the dwarf cache.

Target program:

    #include <sched.h>
    int main(void)
    {
      unsigned int max = 0x6fff, i;
      for (i = 0; i < max; i++)
	sched_yield();
      return 0;
    }

Command line:

	./strace -o /dev/null -k a.out

With the dwarf cache:

    real	0m12.081s
    user	0m3.858s
    sys 	0m8.194s

Without the dwarf cache:

    real	0m22.326s
    user	0m5.218s
    sys		0m16.952s

Signed-off-by: Masatake YAMATO <yamato@redhat.com>