Support aliasing static global nonfunction objects

Support aliasing static global nonfunction objects

Enables code such as:

    #undef NDEBUG
    #include <assert.h>
    #include <stdint.h>
    static int st_x = 42;
    static int st_x_ __attribute((alias("st_x")));
    int main(void){ assert((uintptr_t)&st_x == (uintptr_t)&st_x_); }

which would previously fail with no compiler warnings. The limitation of
this is that the alias must be done (or redone) after an actual definition.
An alias done right after a later overridden tentative declaration won't
work (sufficient for my use case).