perf symbols: Improve debug image search when loading symbols

Changes:
	* Simplification of the main search loop on dso__load()
	* Replace the search with a 2-pass search:
		* First, try to find an image with a proper symtab.
		* Second, repeat the search, accepting dynsym.

A second scan should only ever happen when needed debug images are
missing from the buildid cache or stale, i.e., when the cache is out of
sync.

Currently, the second scan also happens when using separated debug
images, since the caching logic doesn't currently know how to cache
those.  Improvements to the cache behaviour ought to solve that.

Signed-off-by: Dave Martin <dave.martin@linaro.org>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>