Fix for tf_getlongsimtime link error on MinGW/Cygwin.
Fix for tf_getlongsimtime link error on MinGW/Cygwin.

MinGW and Cygwin don't currently support weak function definitions.
For these platforms, make tf_getlongsimtime a strong symbol.

CastMi/iverilog,themperek/iverilog,CastMi/iverilog,themperek/iverilog,themperek/iverilog,CastMi/iverilog