target/i386: Rename helper_fldt, helper_fstt

Change the prefix from "helper" to "do".  The former should be
reserved for those functions that are called from TCG; the latter
is in use within the file already for those functions that are
called from the helper functions, adding a "retaddr" argument.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Claudio Fontana <cfontana@suse.de>
Tested-by: Claudio Fontana <cfontana@suse.de>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210322132800.7470-3-cfontana@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>