Bug 446103 - Memcheck: `--track-origins=yes` causes extreme slowdowns for large mmap...

Bug 446103 - Memcheck: `--track-origins=yes` causes extreme slowdowns for large mmap/munmap.

This patch rewrites the Level 2 origin-tracking cache (ocacheL2) so that
set-address-range-permissions (SARP) operations on it, for large ranges, are
at least a factor of 2.5 x faster.  This is primarily targeted at SARPs in the
range of hundreds to thousands of megabytes.  The Level 1 origin-tracking
cache covers 64MB address space, so SARPs that fit within it are mostly
unaffected.  There are extensive comments in-line.  Changes are:

* Change the Level 2 cache from a single AVL tree (OSet) into 4096 such trees,
  selected by middle bits of the tag, hence "taking out" 12 significant bits
  of search in any given tree.

* For the OCacheLine type, use a union so as to overlay the w32 and descr
  arrays with an array of 64-bit values.  This is used to speed up cases where
  those fields are to be set to zero, or checked against zero.

* Due to the various fast-paths added by this patch, OC_BITS_PER_LINE has
  pretty much been frozen at the current value, 5.

* ocache_sarp_Set_Origins, ocache_sarp_Clear_Origins: deal with large ranges
  in 32-byte steps instead of 4-byte steps.

* MC_(helperc_b_store32), MC_(helperc_b_store16): rewrite these to be (much)
  more efficient.

* fast-return cases for VG_(OSetGen_Lookup) and VG_(OSetGen_Remove) when the
  tree is empty

* a few extra inline hints