Introduce -msmp to select /lib_smp/ on ppc-vx6

The .spec files used for linking on ppc-vx6, when the rtp-smp runtime
is selected, add -L flags for /lib_smp/ and /lib/.

There was a problem, though: although /lib_smp/ and /lib/ were to be
searched in this order, and the specs files do that correctly, the
compiler would search /lib/ first regardless, because
STARTFILE_PREFIX_SPEC said so, and specs files cannot override that.

With this patch, we arrange for the presence of -msmp to affect
STARTFILE_PREFIX_SPEC, so that the compiler searches /lib_smp/ rather
than /lib/ for crt files.  A separate patch for GNAT ensures that when
the rtp-smp runtime is selected, -msmp is passed to the compiler
driver for linking, along with the --specs flags.

for  gcc/ChangeLog

	* config/vxworks-smp.opt: New.  Introduce -msmp.
	* config.gcc: Enable it on powerpc* vxworks prior to 7r*.
	* config/rs6000/vxworks.h (STARTFILE_PREFIX_SPEC): Choose
	lib_smp when -msmp is present in the command line.
	* doc/invoke.texi: Document it.

gcc/config.gcc

@@ -3046,7 +3046,7 @@ powerpc*-wrs-vxworks7r*)
 powerpc-wrs-vxworks*)
 	tm_file="${tm_file} elfos.h gnu-user.h freebsd-spec.h rs6000/sysv4.h"
 	tmake_file="${tmake_file} rs6000/t-fprules rs6000/t-ppccomm rs6000/t-vxworks"
-	extra_options="${extra_options} rs6000/sysv4.opt"
+	extra_options="${extra_options} rs6000/sysv4.opt vxworks-smp.opt"
 	extra_headers="${extra_headers} ppc-asm.h"
 	case ${target} in
           *-vxworksmils*)

gcc/config/rs6000/vxworks.h

@@ -206,7 +206,7 @@ along with GCC; see the file COPYING3.  If not see
 
 #undef  STARTFILE_PREFIX_SPEC
 #define STARTFILE_PREFIX_SPEC						\
- "%{mrtp:%{!shared:/lib/usr/lib/ppc/PPC32/common}}"
+ "%{mrtp:%{!shared:/lib%{msmp:_smp}/usr/lib/ppc/PPC32/common}}"
 
 /* For aggregates passing, use the same, consistent ABI as Linux.  */
 #define AGGREGATE_PADDING_FIXED 0

gcc/config/vxworks-smp.opt

@@ -0,0 +1,25 @@
+; Options for VxWorks configurations where smp/!smp variants of the
+; system libraries are installed in separate locations.
+;
+; Copyright (C) 2023 Free Software Foundation, Inc.
+; Contributed by AdaCore.
+;
+; This file is part of GCC.
+;
+; GCC is free software; you can redistribute it and/or modify it under
+; the terms of the GNU General Public License as published by the Free
+; Software Foundation; either version 3, or (at your option) any later
+; version.
+;
+; GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+; WARRANTY; without even the implied warranty of MERCHANTABILITY or
+; FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+; for more details.
+;
+; You should have received a copy of the GNU General Public License
+; along with GCC; see the file COPYING3.  If not see
+; <http://www.gnu.org/licenses/>.
+
+msmp
+Driver Target RejectNegative
+Select VxWorks SMP C runtimes for linking.

gcc/doc/invoke.texi

@@ -1403,7 +1403,7 @@ See RS/6000 and PowerPC Options.
 -mpointer-size=@var{size}}
 
 @emph{VxWorks Options}
-@gccoptlist{-mrtp  -non-static  -Bstatic  -Bdynamic
+@gccoptlist{-mrtp  -msmp  -non-static  -Bstatic  -Bdynamic
 -Xbind-lazy  -Xbind-now}
 
 @emph{x86 Options}
@@ -32442,6 +32442,13 @@ GCC can generate code for both VxWorks kernels and real time processes
 (RTPs).  This option switches from the former to the latter.  It also
 defines the preprocessor macro @code{__RTP__}.
 
+@opindex msmp
+@item -msmp
+Select SMP runtimes for linking.  Not available on architectures other
+than PowerPC, nor on VxWorks version 7 or later, in which the selection
+is part of the VxWorks build configuration and the library paths are the
+same for either choice.
+
 @opindex non-static
 @item -non-static
 Link an RTP executable against shared libraries rather than static