target/ppc: Implemented xvf16ger*

Implement the following PowerISA v3.1 instructions:
xvf16ger2:   VSX Vector 16-bit Floating-Point GER (rank-2 update)
xvf16ger2nn: VSX Vector 16-bit Floating-Point GER (rank-2 update) Negative
multiply, Negative accumulate
xvf16ger2np: VSX Vector 16-bit Floating-Point GER (rank-2 update) Negative
multiply, Positive accumulate
xvf16ger2pn: VSX Vector 16-bit Floating-Point GER (rank-2 update) Positive
multiply, Negative accumulate
xvf16ger2pp: VSX Vector 16-bit Floating-Point GER (rank-2 update) Positive
multiply, Positive accumulate

Signed-off-by: Lucas Mateus Castro (alqotel) <lucas.araujo@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220524140537.27451-6-lucas.araujo@eldorado.org.br>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>