target/ppc: Fix bugs in VSX_CVT_FP_TO_INT and VSX_CVT_FP_TO_INT2 macros

The patch below fixes a bug in the VSX_CVT_FP_TO_INT and VSX_CVT_FP_TO_INT2
macros in target/ppc/fpu_helper.c where a non-NaN floating point value from the
source vector is incorrectly converted to 0, 0x80000000, or 0x8000000000000000
instead of the expected value if a preceding source floating point value from
the same source vector was a NaN.

The bug in the VSX_CVT_FP_TO_INT and VSX_CVT_FP_TO_INT2 macros in
target/ppc/fpu_helper.c was introduced with commit c3f24257.

This patch also adds a new vsx_f2i_nan test in tests/tcg/ppc64 that checks that
the VSX xvcvspsxws, xvcvspuxws, xvcvspsxds, xvcvspuxds, xvcvdpsxws, xvcvdpuxws,
xvcvdpsxds, and xvcvdpuxds instructions correctly convert non-NaN floating point
values to integer values if the source vector contains NaN floating point values.

Fixes: c3f24257 ("target/ppc: Clear fpstatus flags on helpers missing it")
Resolves: qemu-project/qemu#1941


Signed-off-by: John Platts <john_platts@hotmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>