Update target-ver-syms for MinGW

We do not have/need libm on MiGW
Trivial build fixes for MinGW/druntime
Implement GDC-vararg handling in std.format for MinGW
GDC inline ASM fixes for MinGW
Extern variable _CRT_MT is __gshared
MinGW64 does not provide __imp symbols for InterlockedExchange
Thread, Fiber & GC support for MinGW
Fix main function for MinGW
Add mingw README
Fix TypeInfo name mangling for MinGW