arm: Add missing multilib default values

The arm multilib configuration includes two more parameters which
affect multilib selection, marm/mthumb and mfloat-abi. Without those,
the default multilib selection is mis-specified and the only reason it
works is because '.' is the fall-back path.

Add "marm" and "mfloat-abi=soft" to MULTILIB_DEFAULTS to actually
match when the compiler is run without any target parameters.

This hasn't caused any problems in practice because there are no
non-default multilib options which can be applied to the default
-march target as it has neither an FPU nor any branch protection
support. Specifying another cpu or architecture always sets -marm and
-mfloat-abi and so those multilib configuration don't rely on the
defaults.

Signed-off-by: Keith Packard <keithp@keithp.com>

Author: keith-packard

gcc/config.gcc

@@ -4517,6 +4517,7 @@ case "${target}" in
 					case ${arm_multilib} in
 					aprofile|rmprofile)
 						tmake_profile_file="arm/t-multilib"
+						tm_mlib_file="arm/arm-mlib.h"
 						;;
 					@*)
 						ml=`echo "X$arm_multilib" | sed '1s,^X@,,'`
@@ -4555,6 +4556,7 @@ case "${target}" in
 				# through to the multilib selector
 				with_float="soft"
 				tmake_file="${tmake_file} ${tmake_profile_file}"
+				tm_file="$tm_file $tm_mlib_file"
 				TM_MULTILIB_CONFIG="$with_multilib_list"
 			fi
 		fi

gcc/config/arm/arm-mlib.h

@@ -0,0 +1,22 @@
+/* Arm multilib default option include file.
+
+   Copyright (C) 2023-2024 Free Software Foundation, Inc.
+   Contributed by Arm.
+
+   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/>.  */
+
+#define MULTILIB_DEFAULTS { "marm", "mfloat-abi=soft", }