AppVeyor: build Python in 64-bit mode (GH-8363)

Previously, Python was built in 32-bit mode and so issues specific to
64-bit mode like compiler warnings could be missed.

.github/appveyor.yml

@@ -28,10 +28,10 @@ before_build:
 
 
 build_script:
-  - cmd: PCbuild\build.bat -e
-  - cmd: PCbuild\win32\python.exe -m test.pythoninfo
+  - cmd: PCbuild\build.bat -e -p x64
+  - cmd: PCbuild\amd64\python.exe -m test.pythoninfo
 test_script:
-  - cmd: PCbuild\rt.bat -q -uall -u-cpu -u-largefile -rwW --slowest --timeout=1200 --fail-env-changed -j0
+  - cmd: PCbuild\rt.bat -x64 -q -uall -u-cpu -u-largefile -rwW --slowest --timeout=1200 --fail-env-changed -j0
 environment:
   HOST_PYTHON: C:\Python36\python.exe
 image: