gdb: remove trailing '.' from perror_with_name calls

gdb: remove trailing '.' from perror_with_name calls

I ran into this error while working on AArch64 GDB:

  Unable to fetch VFP registers.: Invalid argument.

Notice the '.:' in the middle of this error message.

This is because of this call in aarch64-linux-nat.c:

  perror_with_name (_("Unable to fetch VFP registers."));

The perror_with_name function take a string, and adds ': <message>' to
the end the string, so I don't think the string that we pass to
perror_with_name should end in '.'.

This commit removes all of the trailing '.' characters from
perror_with_name calls, which give more readable error messages.

I don't believe that any of these errors are tested in the
testsuite (after a little grepping).