cmd/racebuild: install Git and GCC on the Windows builder

Note that compiler-rt/lib/tsan/go/build.bat has been broken since
https://reviews.llvm.org/D28596 (git commit
6ef4606343358c8f0365f7741b5033c42fbabb0e), so we have to use an older
version until it can be fixed.

compiler-rt commit ae08a22cc215448aa3ad5a6fb099f6df77e9fa01 is the
most recent one that builds, but it fails tests (golang/go#22687).

Updates golang/go#24354.
Updates golang/go#22687.

Change-Id: I36ba47fc955111143707224068e687168dbda4ff
Reviewed-on: https://go-review.googlesource.com/112895
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: Bryan C. Mills

cmd/racebuild/racebuild.go

@@ -116,6 +116,12 @@ cp compiler-rt/lib/tsan/go/race_netbsd_amd64.syso go/src/runtime/race
 		Arch: "amd64",
 		Type: "windows-amd64-race",
 		Script: `
+@"%SystemRoot%\System32\WindowsPowerShell\v1.0\powershell.exe" -NoProfile -InputFormat None -ExecutionPolicy Bypass -Command "iex ((New-Object System.Net.WebClient).DownloadString('https://chocolatey.org/install.ps1'))" && SET "PATH=%PATH%;%ALLUSERSPROFILE%\chocolatey\bin"
+choco install git -y
+if %errorlevel% neq 0 exit /b %errorlevel%
+choco install mingw -y
+if %errorlevel% neq 0 exit /b %errorlevel%
+call refreshenv
 git clone https://go.googlesource.com/go
 if %errorlevel% neq 0 exit /b %errorlevel%
 git clone http://llvm.org/git/compiler-rt.git