Use a wider cache for simple function lookups

When we have many individual function lookups, like the nearly 1000 with
syscall.*, each one will iterate every CU in the module (M) and then do a
cache lookup in N entries.  That's a thousand MlogN lookups.

We can instead keep the functions in a module-wide map, and then the
complexity is just a thousand logMN lookups.

Before:
  $ ./run-stap -l 'syscall.**' --vp 01 >/dev/null
  Pass 2: analyzed script: 793 probe(s), 11 function(s), 20 embed(s),
  0 global(s) using 245872virt/147304res/78272shr kb,
  in 1390usr/60sys/1448real ms.

After:
  $ ./run-stap -l 'syscall.**' --vp 01 >/dev/null
  Pass 2: analyzed script: 793 probe(s), 11 function(s), 20 embed(s),
  0 global(s) using 246228virt/147616res/78276shr kb,
  in 720usr/60sys/782real ms.

* dwflpp.cxx (dwflpp::iterate_single_function): Do a simple function
  lookup based on a module-wide cache.
  (dwflpp::mod_function_caching_callback): Helper for above.
* tapsets.cxx (dwarf_query::query_module_functions): Query a single
  function from the module-wide cache.
  (dwarf_query::query_module_dwarf): Use above for simple cases.

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