Added posix function gettimeofday to LibFunc::Func for all platforms but Windows.

*NOTE* In a recent version of posix, they added the restrict keyword to the
arguments for this function. From some spelunking it seems that on some
platforms, the call has restrict on its arguments and others it does not. Thus I
left off the restrict keyword from the function prototype in the comment.
Added support in FunctionAttrs for adding relevant function/argument attributes for the posix call gettimeofday.

This implies annotating it as nounwind and its arguments as nocapture. To be
conservative, we do not annotate the arguments with noalias since some platforms
do not have restrict on the declaration for gettimeofday.