target/i386: DPPS rounding fix

target/i386: DPPS rounding fix

The DPPS (Dot Product) instruction is defined to first sum pairs of
intermediate results, then sum those values to get the final result.
i.e. (A+B)+(C+D)

We incrementally sum the results, i.e. ((A+B)+C)+D, which can result
in incorrect rouding.

For consistency, also change the variable names to the ones used
in the Intel SDM and implement DPPD following the manual.

Based on a patch by Paul Brook <paul@nowt.org>.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>