Introduce #pragma naked

When applied to a function #pragma naked merely disables the "should return
a value" warning for the function. It's intended to be used with functions
that return a value via #emit instead of the normal return statement.

This pragma works only on function definitions, not declarations. It's also
pretty stupid - the function may be defined way after this directive and it
won't stop on things coming in between.

For example, here all declarations between #pragma naked and f() are
effectively ignored:

#pragma naked

new x;       // ignored
forward g(); // ignored
native n();  // ignored

f() {
    // f() becomes naked
}

Note that #pragma naked does not affect generated code in any way, unlike
e.g. __declspec(naked) or __attribute__((naked)) in C/C++ where the compiler
omits the code for the prolog and epilog.