skip fixinc on vxworks7*, amend mkheaders

vxworks7 headers haven't required fixes, and we've decided to avoid
running fixinc on them.

The problem with that is that, with a dummy fixinc, mkheaders wipes
out include-fixed but then multi_dir subdirs are not created again, so
we end up with a limits.h named after each multi_dir, when there are
non-default multilibs.  Oops.

This patch arranges for a dummy fixinc to be created for *-*-vxworks7*
targets, and fixes mkheaders so as to create multi_dir subdirs in
include-fixed after wiping them out, and to copy limits.h so that it
won't take the name that should be of a subdir (unless the multi_dir
is limits.h, but that's hopefully never the case ;-)


for  fixincludes/ChangeLog

	* mkheaders.in: Re-create subdirs, copy limits.h into subdir.
	* mkfixinc.sh: Create dummy fixinc for *-*-vxworks7*.

fixincludes/ChangeLog

@@ -1,3 +1,8 @@
+2020-02-13  Alexandre Oliva <oliva@adacore.com>
+
+	* mkheaders.in: Re-create subdirs, copy limits.h into subdir.
+	* mkfixinc.sh: Create dummy fixinc for *-*-vxworks7*.
+
 2020-01-25  John David Anglin  <danglin@gcc.gnu.org>
 
 	* inclhack.def (hpux_c99_inttypes4): New, add missing SCNuMAX defines.

fixincludes/mkfixinc.sh

@@ -19,6 +19,7 @@ case $machine in
     powerpc-*-rtems*   | \
     powerpcle-*-eabisim* | \
     powerpcle-*-eabi* | \
+    *-*-vxworks7* | \
     *-musl* )
 	#  IF there is no include fixing,
 	#  THEN create a no-op fixer and exit

fixincludes/mkheaders.in

@@ -86,6 +86,7 @@ for ml in `cat ${itoolsdatadir}/fixinc_list`; do
   sysroot_headers_suffix=`echo ${ml} | sed -e 's/;.*$//'`
   multi_dir=`echo ${ml} | sed -e 's/^[^;]*;//'`
   subincdir=${incdir}${multi_dir}
+  ${mkinstalldirs} ${subincdir}
   . ${itoolsdatadir}/mkheaders.conf
   if [ x${STMP_FIXINC} != x ] ; then
 	TARGET_MACHINE="${target}" target_canonical="${target}" \
@@ -100,5 +101,5 @@ for ml in `cat ${itoolsdatadir}/fixinc_list`; do
 	fi
   fi
 
-  cp ${itoolsdatadir}/include${multi_dir}/limits.h ${subincdir}
+  cp ${itoolsdatadir}/include${multi_dir}/limits.h ${subincdir}/limits.h
 done