Improve performance of dwarf3 reader using a hashtable of parsed abbreviations

For each DIE, the dwarf3 reader must know which data elements to read.
These elements are described by an abbreviation.
Re-reading these abbreviations for each DIE is costly as
the location of the needed abbreviation is found by scanning the full
abbv section, which is very costly.
(A small cache of 32 abbv offsets in the abbv section somewhat decreases
the cost, but reading the abbvs is still a hot spot, in particular for
big debug informations).

This patch:
  * adds an hash table of parsed abbreviations
  * all abbreviations for a CU are read in one single scan of the abbv
    section, when the CU header is read
So, with the patch, the di image is not accessed anymore for reading the abbvs
after the CU header parsing.

On a big executable, --read-var-info=yes user cpu changes from
  trunk:      320 seconds
to
  abbv cache: 270 seconds

This further improves on a previous (not committed) abbv cache that
was just caching up to 513 entries in the abbv pos cache and populating
the cache with an initial scan. The user cpu for this version was 285 seconds.

NB: this is some work in anticipation of a following patch that
will add reading dwarf3 inlined information, with the hope to make
this reading fast enough to activate it by default.

Note: on the examples I looked at, all abbreviations were numbered starting
from 1, with no holes. If that would always be the case, then one could use
an xarray of parsed abbreviations rather than an hash table. However,
I found nothing in the dwarf standard that guarantees that abbreviations
are numbered from 1. So, the hash table.
  




git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14035 a5019735-40e9-0310-863c-91ae7b9d1cf9