Use tcmalloc via ld_preload

Fixes trac issue [#10271](http://trac.mantidproject.org/mantid/ticket/10271)

Testing: This only affects Linux platforms. Both the Debian and RPM packages have been updated to use a `launch_mantidplot.sh` script that preloads `tcmalloc.so`. To check that tcmalloc is loaded you can examine the output of
```
$ LD_DEBUG=libs bin/launch_mantidplot.sh > lddebug.log 2>&1
```
that is contained in the lddebug.log file. It should have references to tcmalloc, whereas
```
$ ldd bin/MantidPlot | grep tcmalloc
```
should produce nothing.

Ideally the packages should be checked by installing them, checking MantidPlot & uninstalling them, ensuring no errors occur.