ada_print_floating: Remove use of statically sized buffer.

ada_print_floating: Remove use of statically sized buffer.

ada_print_floating declares a char buffer with a size that we're hoping
to always be large enough to hold any string representation of a float
value.  But that's not really necessary, and also forces us to create
a small wrapper (ui_memcpy) to perform the extraction from a temporary
stream into this buffer.  This patches fixes both issues by relying on
ui_file_xstrdup.  This forces us to make a few adjustments that are
minor in nature, as we now need to defer the cleanup to the end of
the function.

gdb/ChangeLog:

        * ada-valprint.c (ui_memcpy): Delete.
        (ada_print_floating): Update documentation.  Add empty line
        between between function documentation and implementation.
        Delete variable "buffer".  Use ui_file_xstrdup in place of
        ui_file_put.  Minor adjustments following this change.