CFI reader: make reading of 'encoded addresses' be much more in

CFI reader: make reading of 'encoded addresses' be much more in
line with the DWARF3 spec and also with binutils/readelf.c:

- Update some comments

- Get rid of kludge_then_addDiCfSI; apparently no longer needed

- Pass the SegInfo's text_bias around in the AddressDecodingInfo,
  so that ...

- read_encoded_Addr can set 'base' to the text_bias when handling
  DW_EH_PE_absptr.  This is the central change of this commit and
  appears (to me) to be what DWARF3 requires.  (The spec is less
  than clear ..)

- don't use read_encoded_Addr to read the FDE arange field since
  read_encoded_Addr's adding-on of a 'base' value is meaningless
  here - the arange is not an address, but a value saying how many
  bytes the FDE covers.  Instead just read a little-endian value of
  the right size.  This is in accordance with DWARF3 and with
  readelf.c.  Add new function read_le_encoded_literal to make
  this possible.

I believe this is all correct, and it's certainly much better than it
was.  But given that the DWARF3 spec isn't as formal as it should be,
it's hard to be sure.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@6611