gdb/gdbarch: fix a dot-space-space in generated files

This is a very small patch to straighten out dot-space-space in these
comments in the gdbarch generated files:

-  /* Skip verify of short_bit, invalid_p == 0 */
+  /* Skip verify of short_bit, invalid_p == 0.  */

There is no functional change after this commit.

Approved-By: Andrew Burgess <aburgess@redhat.com>