LibraryCallMonitor: add support for ELF files

[vitalych]

LIbraryCallMonitor is currently limited to Windows PE files because of the limitations of the underlying file parsers. It is possible to fix it as follows:

1. Add a getExports() call to ELFFile.h [1]. Currently, only PEFile.h supports getting exported symbols [2]. You will have to parse the ELF file to figure out the addresses of the exported functions.
2. Adapt the cast in LibraryCallMonitor so that it also works with ELF files [3]
3. We may need to add support for lazy binding in LibraryCallMonitor, which is pretty common on Linux. Otherwise, the program has to built with -fno-plt.

[1] https://github.com/S2E/s2e/blob/master/libvmi/include/vmi/ELFFile.h
[2] https://github.com/S2E/s2e/blob/master/libvmi/include/vmi/PEFile.h#L127
[3] https://github.com/S2E/s2e/blob/master/libs2eplugins/src/s2e/Plugins/ExecutionMonitors/LibraryCallMonitor.cpp#L207