softfloat: add int128_to_float128

Based on parts_sint_to_float, implements int128_to_float128 to convert a
signed 128-bit value received through an Int128 argument.

Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Message-Id: <20220330175932.6995-5-matheus.ferst@eldorado.org.br>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>