Previously we started the Go runtime from a JNI function call, which
eventually called the program's main function. Now the runtime is
initialized by an ELF initialization function as a c-shared library,
and the program's main function is not called. So now we export main
so it can be called from JNI.

This is necessary for all-Go apps because unlike a normal shared
library, the program loading the library is not written by or known
to the programmer. As far as they are concerned, the .so is
everything. In fact the same code is compiled for iOS as a normal Go
program.

Change-Id: I61c6a92243240ed229342362231b1bfc7ca526ba
Reviewed-on: https://go-review.googlesource.com/9015Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>