Fixed bugs found on Ubuntu 12.04.

* compilers.jam: Removed `pushd` and `popd` because they are bash-specific
  builtins.
* jamroot: Removed `unspecified` value from `<intro-address-model>` feature.
  The default value for the feature is the system default bits (e.g., `64`
  for x86_64-*, `32` for i686-*, and so on).
* gcc/jamfile: Removed `ln -s lib lib64` because the convention of library
  directories differs from distribution to distribution and `lib`
  directory cannot be assumed to be the directory for 64-bit libraries.
* icu4c/jamfile: Removed `--enable-threads` or `--disable-threads` from the
  command line options for ICU4C configure script because they are no longer
  valid in recent versions of ICU4C.
* openmpi/jamfile: Removed `<intro-address-model>unspecified` property.
* boost/jamfile: Likewise.