[rfc] Replace current_gdbarch in *mips*

[Joel Brobecker]

> this patch replaces most of the current_gdbarch's found in *mips*.
> These replacements are mostly mechanical changes. Unfortunately I dont
> have access to MIPS (linux, bsd, remote) machines.  This patch is
> hence only "tested" by building GDB with --enable-targets=all.
> 
> Maybe someone from you can run the testsuite with the patch attached? 
> Thanks in advance.

I will test the patch on mips-irix. This is not the platform where
we get the best results, so if someone has access to a mips-linux
matchine, I think we would get more meaningful results.

-- 
Joel