add -Wl,-z,relro,-z,now to hardening ldflags

See: https://www.redhat.com/en/blog/hardening-elf-binaries-using-relocation-read-only-relro

as a happy side effect this reduces dirty pages as measured by

pmap -px PID|grep 'rw.--'|grep -v anon|awk '{ sum+=$4 } END { print sum }'

for a --with-distro=CPLinux-LOKit build and spawned kit calc process from
2588 to 2352 pages

Change-Id: I86b3ae025300907a240affd6d9a3d36d2eecbfb5
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/187469
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>
Tested-by: Jenkins

config_host.mk.in

@@ -196,6 +196,7 @@ export ENABLE_GTK4=@ENABLE_GTK4@
 export ENABLE_GTKTILEDVIEWER=@ENABLE_GTKTILEDVIEWER@
 export DISABLE_GUI=@DISABLE_GUI@
 export ENABLE_HARDENING_FLAGS=@ENABLE_HARDENING_FLAGS@
+export HARDENING_LDFLAGS=@HARDENING_LDFLAGS@
 export HARDENING_CFLAGS=@HARDENING_CFLAGS@
 export HARDENING_OPT_CFLAGS=@HARDENING_OPT_CFLAGS@
 export ENABLE_HEADLESS=@ENABLE_HEADLESS@

configure.ac

@@ -7795,9 +7795,20 @@ dnl ===================================================================
 dnl GCC features
 dnl ===================================================================
 HAVE_GCC_STACK_CLASH_PROTECTION=
+HARDENING_LDFLAGS=
 HARDENING_CFLAGS=
 HARDENING_OPT_CFLAGS=
 if test "$GCC" = "yes" -o "$COM_IS_CLANG" = TRUE; then
+
+    AC_MSG_CHECKING([for full RELRO linker support])
+    save_LDFLAGS=$LDFLAGS
+    LDFLAGS="$LDFLAGS -Wl,-z,relro,-z,now"
+    AC_LINK_IFELSE(
+        [AC_LANG_PROGRAM(, [[return 0;]])],
+        [AC_MSG_RESULT([yes]); HARDENING_LDFLAGS="$HARDENING_LDFLAGS -Wl,-z,relro,-z,now"],
+        [AC_MSG_RESULT([no])])
+    LDFLAGS=$save_LDFLAGS
+
     AC_MSG_CHECKING([whether $CC_BASE supports -grecord-gcc-switches])
     save_CFLAGS=$CFLAGS
     CFLAGS="$CFLAGS -Werror -grecord-gcc-switches"
@@ -7996,6 +8007,7 @@ fi
 AC_SUBST(HAVE_GCC_AVX)
 AC_SUBST(HAVE_GCC_BUILTIN_ATOMIC)
 AC_SUBST(HAVE_GCC_STACK_CLASH_PROTECTION)
+AC_SUBST(HARDENING_LDFLAGS)
 AC_SUBST(HARDENING_CFLAGS)
 AC_SUBST(HARDENING_OPT_CFLAGS)
 

solenv/gbuild/platform/unxgcc.mk

@@ -72,6 +72,7 @@ ifeq (,$(DISABLE_DYNLOADING))
 gb_LinkTarget_LDFLAGS += \
 	-Wl,-rpath-link,$(SYSBASE)/lib:$(SYSBASE)/usr/lib \
 	-Wl,-z,combreloc \
+	$(if $(ENABLE_HARDENING_FLAGS),$(HARDENING_LDFLAGS)) \
 
 endif