Rework MIPS macro relaxation, fix string merging bug

[Thiemo Seufer]

Richard Sandiford wrote:
[snip]
>   - It makes it possible to relax n64 NO_PIC sequences into GP-relative
>     sequences.
> 
> However, the patch doesn't try to do any of these.  In particular,
> it keeps the macro_build() argument, even though its value is no
> longer used.  I thought this was better than drowning the main change
> (which is quite finicky) in a sea of other stuff.  I'll try to clean
> things up in a follow-on patch.

You may however adjust the comment about GP optimization in the
three n64 NO_PIC cases.

[snip]
> Also tested on mips64vrel-elf, which, with all its multlibs, should
> cover all the NO_PIC cases.  Tested against the binutils testsuite
> on mips64{,el}-linux-gnu and mips64{,el}-elf.  OK to install?

Ok for the mips-specific part.


Thiemo