MIPS constant tweaks, add -fsection-anchor support

[Richard Sandiford]

Richard Sandiford <richard@codesourcery.com> writes:
> The patch also adds section anchor support.  This probably needs a bit more
> tuning for MIPS16 and TARGET_ABICALLS[*]. [...]
>
> [*] in the latter case because of the two-instruction access sequence
>     for local symbols.  It would probably be better to use %got_disp
>     for anchor symbols if TARGET_NEWABI.

It actually turned out a bit better than I thought.  Attached are some
CSiBE comparisons for the three mips64-linux-gnu ABIs, all compiled
with -Os.  The first column is -fno-section-anchors and the second
is -fsection-anchors, both produced from a patched compiler.

The patch has now been bootstrapped & regression-tested on mips64-linux-gnu,
which showed up that:

> 	* config/mips/mips.h: Protect externs with !IN_LIBGCC.

...this was wrong for objc: it should have been USED_FOR_TARGET instead.

There were no objections to the patch, so I applied it with that change.

Richard

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