PR/56148: Andreas Gustafsson: lib/libc/stdio/t_printf:snprintf_float test
randomly fails.
Add checks to all places where lshift is called because it can return NULL

Sprinkle some more const in.

Merge the new gdtoa: Note that the built-in tests fail the same way as with
the old gdtoa.

Don't coredump on out of memory conditions. This solution leaks, but gdtoa
is too complicated to fix. Try printf %99999999999.9999999999f 2

Import gdtoa-2005-09-23; test suite will be imported separately.