Fix compile time warnings in tc-mips.c

[Maciej W. Rozycki]

Hi Nick,

> > Anyway, thanks for the fix -- I must have not looked for a possible
> > suitable function well enough when implementing these bits. 
> 
> If it helps, I ran across this problem whilst building a "mips-ecoff"
> toolchain using a GCC 3.4 based compiler on a 32bit host.

 Of course -- I must have paid attention to ELF targets only.  I think 
hardly anyone builds a pure ECOFF MIPS toolchain these days, so yours was 
quite a clever test.

  Maciej