build: Pass -64 to goversioninfo on amd64

[imsodin]

Trying to build syncthing on windows with mingw-w64 I got the following error:

 // c:\go\pkg\tool\windows_amd64\link.exe: running gcc failed: exit status 1
// C:/Program Files/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/8.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: i386 architecture of input file `C:\Users\2007p\AppData\Local\Temp\go-link-332898159\000000.o' is incompatible with i386:x86-64 output
// collect2.exe: error: ld returned 1 exit status

Passing -w64 to goversioninfo fixed that. However I am not sure which arch setting is relevant here: The host system (probably not), the c++ compiler binary or go target architecture. I would expect the latter (and proposed the change like that), however CI right now is building both 32 and 64 bit binaries for windows and it works, without the -64 flag.

[AudriusButkevicius]

I don't see why we even need a C compiler. Pulling in badger introduced it, so I think we should just build with CGO_ENABLED=0 on windows.

[imsodin]

Hmm, ok, didn't realize badger was the only thing using cgo. Annoying... Luckily that cgo requirement is hopefully going away soon on badger (dgraph-io/badger#1162), I wouldn't want to fork it to remove cgo...

But yeah, not having to go through the hassle of getting a C compiler setup in the first place is nice, so lets do CGO_ENABLED=0 instead.