target-ppc: Add VSX xmax/xmin Instructions

target-ppc: Add VSX xmax/xmin Instructions

This patch adds the VSX floating point maximum and minimum
instructions:

  - xsmaxdp, xvmaxdp, xvmaxsp
  - xsmindp, xvmindp, xvminsp

Because of the Power ISA definitions of maximum and minimum
on various boundary cases, the standard softfloat comparison
routines (e.g. float64_lt) do not work as well as one might
think.  Therefore specific routines for comparing 64 and 32
bit floating point numbers are implemented in the PowerPC
helper code.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Alexander Graf <agraf@suse.de>