Fix build on Windows/VC++

- Stop defining custom architectures because VisualCpp toolchain doesn't
  allow it. Reference to built-in architectures 'x86' and 'x86_64' since
  they are supported by all toolchains.
- Remove 'local_arch' from platforms. For unsupported platform, we just
  do not specify the target.
- Target no more than one platform at a time. This simplifies the build
  script a lot.
- Remove the TARGET_ARCHS environment variable. Add system property
  ``arch`` to override ``osdetector.arch``.
- Add ``vc.disable`` to override the default choice of VisualCpp on
  Windows.
- Add ``vc.`` prefix to the properties that are only used with VC++
diff --git a/build.gradle b/build.gradle
index 37c7764..00afd94 100644
--- a/build.gradle
+++ b/build.gradle
@@ -69,7 +69,7 @@
 
         def exeSuffix = osdetector.os == 'windows' ? ".exe" : ""
         protocPluginBaseName = 'protoc-gen-grpc-java'
-        javaPluginPath = "$rootDir/compiler/build/binaries/java_pluginExecutable/local_arch/$protocPluginBaseName$exeSuffix"
+        javaPluginPath = "$rootDir/compiler/build/binaries/java_pluginExecutable/$protocPluginBaseName$exeSuffix"
     }
 
     dependencies {