The macOS catalina compiler produces an error for __int128_t
as the type for allocation with SSE inline asm constraint.
Create a new X86Int128Union type and use the vector type for
all SSE register inputs and outputs.
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
8a2bd74787