ci: scripts to setup mingw build environment

(cherry picked from commit 9e588060)

ci/setup-mingw-amd64.ps1

+Set-StrictMode -Version Latest
+
+$ErrorActionPreference = "Stop"
+$PSDefaultParameterValues['*:ErrorAction'] = 'Stop'
+
+[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
+
+[Reflection.Assembly]::LoadWithPartialName("System.IO.Compression.FileSystem");
+
+Write-Host "##############################################################################"
+Write-Host "## Downloading mingw"
+Write-Host "##############################################################################"
+
+$mingw_uri = "https://bintray.com/libgit2/build-dependencies/download_file?file_path=mingw-w64-x86_64-8.1.0-release-win32-seh-rt_v6-rev0.zip"
+$platform = "x86_64"
+
+$wc = New-Object net.webclient
+$wc.Downloadfile($mingw_uri, "${Env:TEMP}/mingw-${platform}.zip")
+
+[System.IO.Compression.ZipFile]::ExtractToDirectory("${Env:TEMP}/mingw-${platform}.zip", $Env:TEMP)

ci/setup-mingw-x86.ps1

+Set-StrictMode -Version Latest
+
+$ErrorActionPreference = "Stop"
+$PSDefaultParameterValues['*:ErrorAction'] = 'Stop'
+
+[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
+
+[Reflection.Assembly]::LoadWithPartialName("System.IO.Compression.FileSystem");
+
+Write-Host "##############################################################################"
+Write-Host "## Downloading mingw"
+Write-Host "##############################################################################"
+
+$mingw_uri = "https://bintray.com/libgit2/build-dependencies/download_file?file_path=mingw-w64-i686-8.1.0-release-win32-sjlj-rt_v6-rev0.zip"
+$platform = "x86"
+
+$wc = New-Object net.webclient
+$wc.Downloadfile($mingw_uri, "${Env:TEMP}/mingw-${platform}.zip")
+
+[System.IO.Compression.ZipFile]::ExtractToDirectory("${Env:TEMP}/mingw-${platform}.zip", $Env:TEMP)