Introduce generic syscall interface

This allows us to write syscall implementation in JS.

This mean, for example, that we can compile full libc code, and
implement only syscalls.

This prevent us from reinventing wheel over and over while implementing
some "new" libc function.

When we implementing, say, getpid() function, we really implement
getpid() syscall. But some logic in libc in that case is thrown out.
This is wrong. We should not either throw or re-implement this logic.

Stay tuned.
sigaction() stub implementation through syscall
debug emcc