Fix host_name and target_name generation by common/create-version.sh.

Fix host_name and target_name generation by common/create-version.sh.

This new script has one small snafoo, which prevented the $host_alias
and $target_alias from being expanded during the generation of the
version.c file.  As a result, the version info yields:

    This GDB was configured as "--host=$host_alias --target=$target_alias".
                                       ^^^^^^^^^^^          ^^^^^^^^^^^^^

This patch fixes this issue.

gdb/ChangeLog:

        * common/create-version.sh: Fix expansion of $host_alias
        and $target_alias in generation of HOST_NAME and TARGET_NAME
        (resp.).