global vars and symbol visibility for mips32/elf

[H.J. Lu]

> 
> While for the linker, we use a variant of the Linux linker distributed
> of H.J.Lu, which is a somehow modified GNU ld. There again somebody did 
> the work and put in warning messages for the case that commons were
> linked against library functions.  Again, this person obviously knew
> what the problem was and deliberately decided to issue a warning instead
> of preventing the symbol to match.

I modified BFD toa issue a warning when the symbol'ss type changes.
It doesn't necessarily mean an error, especially when a read-only 
symbols are put into the .text section. But in some cases, the
warning helps when you have

foo ()
{
}

in a shared library and

int foo = 0;

in the application.


H.J.