Fix MinGW shared, PIC and rdynamic flags.
Fix MinGW shared, PIC and rdynamic flags.

A native MinGW compile was not setting these to the correct
value. This patch makes them match what Cygwin uses. Adding
the -Wl,--enable-auto-image-base should make vvp load
slightly faster since it loads multiple DLL files.

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