[PATCHv2][PR target/81535] Fix tests on Power

[Yury Gribov]

Hi all,

This is an updated version of patch for PR 81535.  The patch was last
discussed in November.  The new version addresses Segher's comments in
https://gcc.gnu.org/ml/gcc-patches/2017-08/msg00558.html

The patch was tested on
  x86_64-pc-linux-gnu
  powerpc64-unknown-linux-gnu
  powerpc64le-unknown-linux-gnu
by running
  $ make check-gcc RUNTESTFLAGS='powerpc.exp=pr79439-*.c dg.exp=pr56727-*.c '
  $ make check-c RUNTESTFLAGS='powerpc.exp=pr79439-*.c
dg.exp=pr56727-*.c --target_board="unix/-m32"'
(I didn't perform full testing as I don't change anything in compiler).

FTR here's an excerpt from
https://gcc.gnu.org/ml/gcc-patches/2017-11/msg02231.html which
explains the reasoning behind PR 56727 (which introduced regression in
PR 81535):

>> Is it correct that current GCC does not do the call via the PLT?
>
> Well, it was decided in
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56727 that it would be a
> valid optimization because the only way to expose the difference would
> be through dlsym hackery.  Note that original PowerPC use-case
> (reported in https://sourceware.org/bugzilla/show_bug.cgi?id=21116)
> would benefit from this optimization as because PLT call + indirection
> would be replaced by normal PC-relative call.

-Y
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