bind, cmd: generate complete standalone bindings from gobind

The gobind and gomobile bind tools have historically overlapped:
gobind outputs generated bindings, and gomobile bind will generate
bindings before building them. However, the gobind bindings were
never used for building and thus allowed to not be complete.

To simplify version control, debugging, instrumentation and build
system flexibility, this CL upgrades the gobind tool to be the
canonical binding generator and change gomobile bind to use gobind
instead of its own generator code.

This greatly simplifies gomobile bind, but also paves the way to skip
gomobile bind entirely. For example:

$ gobind -outdir=$GOPATH golang.org/x/mobile/example/bind/hello
$ GOOS=android GOARCH=arm64 CC=<ndk-toolchain>/bin/clang go build -buildmode=c-shared -o libgobind.so gobind
$ ls libgobind.*
libgobind.h  libgobind.so

The same applies to iOS, although the go build command line is more
involved.

By skipping gomobile it is possible to freely customize the Android
or iOS SDK level or any other flags not supported by gomobile bind.
By checking in the generated source code, the cost of supporting
gomobile in a custom build system is also decreased.

Change-Id: I59c14a77d625ac1377c23b3213672e0d83a48c85
Reviewed-on: https://go-review.googlesource.com/99316
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

bind/java/seq_android.c.support

@@ -19,6 +19,10 @@
 
 #define NULL_REFNUM 41
 
+// initClasses are only exported from Go if reverse bindings are used.
+// If they're not, weakly define a no-op function.
+__attribute__((weak)) void initClasses(void) { }
+
 static JavaVM *jvm;
 // jnienvs holds the per-thread JNIEnv* for Go threads where we called AttachCurrentThread.
 // A pthread key destructor is supplied to call DetachCurrentThread on exit. This trick is