[build] Support --sysroot with Solaris ld

[Rainer Orth]

The Solaris linker recently gained sysroot support.  The following patch
enables that, although there isn't much to do:

* Until recently, ld --help output went to stderr, not being caught by
  gcc/configure's tests which only checked stdout.  However, older ld
  versions still differ here and libtool long has been checking both
  stdout and stderr, so this seems a pretty obvious change to me.

  The only point worth mentioning is that I've guarded the --as-needed
  check for non-GNU ld: before, the native Solaris -z ignore/-z record
  forms were used, now this would use the compat options
  --as-needed/--no-as-needed which occur in the --help output.

* While Solaris ld *does* support --sysroot for gld compatibility, we've
  always preferred the native forms of the options, -z sysroot in this
  case, which the sol2.h part of the patch implements.

Tested in i386-pc-solaris2.12 and sparc-sun-solaris2.12 builds with both
ld and gld, checking that auto-host.h has no unexpected changes.

Also tested with i386-pc-solaris2.12 x sparc-sun-solaris2.12 and
sparc-sun-solaris2.12 x i386-pc-solaris2.12 crosses with both ld (which
has been a cross-linker for quite some time) and gld, checking that -z
sysroot/--sysroot was used as expected.

Ok for mainline?

	Rainer

-- 
-----------------------------------------------------------------------------
Rainer Orth, Center for Biotechnology, Bielefeld University


2017-05-16  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	* configure.ac (gcc_cv_ld_static_dynamic): Also check stderr for
	$gcc_cv_ld --help output.
	(gcc_cv_ld_demangle): Likewise.
	(gcc_cv_ld_eh_frame_hdr): Likewise.
	(gcc_cv_ld_pie): Likewise.
	(gcc_cv_ld_as_needed): Likewise.  Prefer native forms unless $gnu_ld.
	(gcc_cv_ld_buildid): Likewise.
	(gcc_cv_ld_sysroot): Likewise.
	(ld_bndplt_support): Likewise.
	(ld_pushpopstate_support): Likewise.
	* configure: Regenerate.
	* config/sol2.h [!USE_GLD] (SYSROOT_SPEC): Define.

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