This change makes use of a VSX instruction to generate the
float 0 value instead of generating a constant in memory and
loading it from there.

This uses 1 instruction instead of 2 and avoids a memory reference.
in the +0 case, uses 2 instructions in the -0 case but avoids
the memory reference.

Since this is done in the assembler for ppc64x, an update has
been made to the assembler test.

Change-Id: Ief7dddcb057bfb602f78215f6947664e8c841464
Reviewed-on: https://go-review.googlesource.com/c/139420Reviewed-by: Michael Munday <mike.munday@ibm.com>