cmd/cgo: don't use syscall.Errno type as errno return on Windows

Apologies in advance If I'm explaining anything poorly.

What did you do?

I made an Go binding library for a static C lib, then wrote a go application to use that. During the course of developing that application, I managed to trigger the following error:

The process cannot access the file because another process has locked a portion of the file.

I have a stripped down testable chunk of code (complete with a more in depth readme) here: https://github.com/technicalviking/cgotest

I'd like to stress: I'm not looking to debug the code itself. I know the outputs are not what I expected (casting the C **double variable back to [][]float64 in 'extractOutputs' shows a significant number of NaN entries, but meh), but rather why the functionality puts the code in a state where a windows error populates the error return value of the go reference to the C function in this case.

EDIT TO INVESTIGATION

On further digging I found that calling sqrt on a negative number, even in a function defined in the preamble, is sufficient to trigger this behavior. The test code referenced in this issue has been updated accordingly.

What did you expect to see?

No errors

What did you see instead?

calling the c function using the mechanism doResponse, doError = C.bridgeIndicatorFunction(...) to leverage a C function pointer results in doError containing the following value: "The process cannot access the file because another process has locked a portion of the file."

System details

go version go1.9.2 windows/amd64
GOARCH="amd64"
GOBIN=""
GOEXE=".exe"
GOHOSTARCH="amd64"
GOHOSTOS="windows"
GOOS="windows"
GOPATH="C:\Users\dmurker\Documents\Dev\DM\Go Projects"
GORACE=""
GOROOT="C:\Program Files Dev\Go"
GOTOOLDIR="C:\Program Files Dev\Go\pkg\tool\windows_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\D-DANI~1\AppData\Local\Temp\1\go-build223821631=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOROOT/bin/go version: go version go1.9.2 windows/amd64
GOROOT/bin/go tool compile -V: compile version go1.9.2
gdb --version: GNU gdb (GDB) 7.10.1