mmap_cache: move code for searching a mmap cache from unwind

mmap_cache: move code for searching a mmap cache from unwind

print_stack_frame function in unwind.c searches a mmap entry in mmap
cache.  The found entry is then used for unwinding.  However, a function
searching for a mmap entry may be useful for other purposes than
unwinding.

This change re-factors the function; code for searching an entry is
now defined as a stand-alone function named mmap_cache_search.

* defs.h (mmap_cache_search): New function prototype.
print_stack_frame.
* mmap_cached.c (mmap_cache_search): New function derived from
print_stack_frame.
* unwind.c (print_stack_frame): Use it.

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