remove unsupported --libc=klee option from help message

[MichalHe]

The help message mentions the possibility to use --libc=klee option, however, no such option is passed to getopt, nor is it handled while iterating over supplied options. Using this option causes symbiotic to exit with ERROR: option --libc not recognized.

[MichalHe]

Also, the following options are currently parsed, however, they are undocumented.

--cc
--executable-witness
--gen-c
--link
--no-link-undefined
--no-witness
--target
--target-settings
--unroll

I am not sure whether they are undocumented on purpose, but if it is the case, feel free to ignore this comment.