Support for mixed mode applications.

On a 64-bit machine, DMTCP can be used to checkpoint a
computation consisting of both 64-bit and 32-bit applications.

The DMTCP binaries (dmtcp_launch, dmtcp_coordinator, dmtcp_restart, etc.) are
compiled and built only for the 64-bit architecture.  These binaries can launch
processes for 32-bit applications as well. An exception is mtcp_restart, which
must be compiled and build for each supported architecture.

The libraries (libdmtcp.so, libdmtcp_pid.so, etc.) are built separately for all
supported architectures.

The installation goes as follows:
<prefix>/bin: binaries for primary architecture
<prefix>/lib/dmtcp: libraries for primary architecture
<prefix>/lib/dmtcp/32/bin: mtcp_restart binary for given secondary arch.
<prefix>/lib/dmtcp/32/lib/dmtcp: libraries for given secondary arch.

Rest of the data files (headers, docs, manpages, etc) go to their usual
locations.

A. Compiling DMTCP for multi-arch support:
=========================================

1. Do the usual 64-bit compilation
   ./configure
   make
   make install
2. Do the 32-bit compilation (with -m32)
   ./configure --enable-m32
   make clean
   make
   make install

B. Handling dlsym offset:
========================
DMTCP relies on dlsym offset (calculated as the relative offset of dlsym from
dlinfo in libdl.so) for calling the libdl:dlsym() function. This is to allow
the plugins to put wrappers around dlsym() function itself.

As can be seen, the dlsym offset may be different for 32-bit and 64-bit
versions of libdl.so. To handle this, we calculate the offset for both
architectures before hand (while executing in dmtcp_launch, and dmtcp_restart)
and set the environment variables DMTCP_DLSYM_OFFSET (and
DMTCP_DLSYM_OFFSET_M32). These offsets are also stored in SharedData area (see
shareddata.h).

While exec'ing a new program, the process can not rely on the current value
of the env vars as the values might be outdated. Thus, in prepareForExec(), the
env vars are updated from the current values in SharedData.