Remove obsolete Solaris 11.3 support

Support for Solaris 11.3 had already been obsoleted in GCC 13.  However,
since the only Solaris system in the cfarm was running 11.3, I've kept
it in tree until now when both Solaris 11.4/SPARC and x86 systems have
been added.

This patch actually removes the Solaris 11.3 support.  Apart from
several minor simplifications, there are two more widespread changes:

* In Solaris 11.4, libsocket and libnsl were folded into libc, so
  there's no longer a need to link them explictly.

* Since Solaris 11.4, Solaris includes all crts needed by gcc (like
  crt1.o and gcrt1.o) with the base system.  All workarounds to provide
  fallbacks can thus go.

Bootstrapped without regressions on i386-pc-solaris2.11 and
sparc-sun-solaris2.11 (as/ld, gas/ld, and gas/gld) as well as Solaris
11.3/x86 to ascertain that version is actually rejected.

2024-04-30  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	c++tools:
	* configure.ac (ax_lib_socket_nsl.m4): Don't sinclude.
	(AX_LIB_SOCKET_NSL): Don't call.
	(NETLIBS): Remove.
	* configure: Regenerate.
	* Makefile.in (NETLIBS): Remove.
	(g++-mapper-server$(exeext)): Remove $(NETLIBS).

	gcc:
	* config.gcc: Move *-*-solaris2.11.[0-3]* to unsupported list.
	<*-*-solaris2*> (default_use_cxa_atexit): Set unconditionally.
	* configure.ac (AX_LIB_SOCKET_NSL): Don't call.
	(NETLIBS): Remove.
	(gcc_cv_ld_aligned_shf_merge): Remove.
	(hidden_linkonce) <i?86-*-solaris2* | x86_64-*-solaris2*>: Remove.
	(gcc_cv_target_dl_iterate_phdr) <*-*-solaris2*>: Always set to yes.
	* Makefile.in (NETLIBS): Remove.
	* configure, config.in, aclocal.m4: Regenerate.
	* config/sol2.h: Don't check HAVE_SOLARIS_CRTS.
	(STARTFILE_SPEC): Remove !HAVE_SOLARIS_CRTS case.
	[USE_GLD] (LINK_EH_SPEC): Remove TARGET_DL_ITERATE_PHDR guard.
	* config/i386/i386.cc (USE_HIDDEN_LINKONCE): Remove guard.
	* varasm.cc (mergeable_string_section): Remove
	HAVE_LD_ALIGNED_SHF_MERGE handling.
	(mergeable_constant_section): Likewise.

	* doc/install.texi (Specific,i?86-*-solaris2*): Reference Solaris
	11.4 only.
	(Specific, *-*-solaris2*): Document Solaris 11.3 removal.  Remove
	11.3 references and caveats.  Update for 11.4.

	gcc/cp:
	* Make-lang.in (cc1plus$(exeext)): Remove $(NETLIBS).

	gcc/objcp:
	* Make-lang.in (cc1objplus$(exeext)): Remove $(NETLIBS).

	gcc/testsuite:
	* lib/target-supports.exp (check_effective_target_pie): Always
	enable on *-*-solaris2*.

	libgcc:
	* configure.ac <*-*-solaris2*> (libgcc_cv_solaris_crts): Remove.
	* config.host <*-*-solaris2*>: Remove !libgcc_cv_solaris_crts
	support.
	* configure, config.in: Regenerate.
	* config/sol2/gmon.c (internal_mcount) [!HAVE_SOLARIS_CRTS]: Remove.
	* config/i386/sol2-c1.S, config/sparc/sol2-c1.S: Remove.
	* config/sol2/t-sol2 (crt1.o, gcrt1.o): Remove.

	libstdc++-v3:
	* testsuite/lib/dg-options.exp (add_options_for_net_ts)
	<*-*-solaris2*>: Don't link with -lsocket -lnsl.