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

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.