adjust src-release following the renaming of gdb/common/ to gdb/gdbsupport/

A recent change renamed the common/ directory into gdbsupport/ in gdb.
This causes problems in the getver function in the src-release script
which doesn't find the create-version.sh script anymore. As a result,
it falls back on using the version.in file verbatim, meaning that
the "DATE" placeholder doesn't get replaced with the snapshot date,
and the "-git" suffix doesn't get stripped. More precisely, we get
snapshots called "gdb-8.3.50-DATE-git.tar" instead of (e.g.)
"gdb-8.3-20190712.tar".

This commit fixes the issue by adding support for this situation.
I left the support for $tool/common/create-version.sh, because
the sim still uses that directory structure.

ChangeLog:

        * src-release (getver): If $tool/gdbsupport/create-version.sh
        exists, use that to determine the version number.

ChangeLog

@@ -1,3 +1,8 @@
+2019-07-13  Joel Brobecker  <brobecker@adacore.com>
+
+	* src-release (getver): If $tool/gdbsupport/create-version.sh
+	exists, use that to determine the version number.
+
 2019-06-21  Andreas Schwab  <schwab@linux-m68k.org>
 
 	* src-release.sh (GDB_SUPPORT_DIRS): Add gnulib.

src-release.sh

@@ -61,6 +61,10 @@ getver()
 	$tool/common/create-version.sh $tool 'dummy-host' 'dummy-target' VER.tmp
 	cat VER.tmp | grep 'version\[\]' | sed 's/.*"\([^"]*\)".*/\1/' | sed 's/-git$//'
         rm -f VER.tmp
+    elif test -f $tool/gdbsupport/create-version.sh; then
+	$tool/gdbsupport/create-version.sh $tool 'dummy-host' 'dummy-target' VER.tmp
+	cat VER.tmp | grep 'version\[\]' | sed 's/.*"\([^"]*\)".*/\1/' | sed 's/-git$//'
+        rm -f VER.tmp
     elif test -f $tool/version.in; then
 	head -1 $tool/version.in
     else