rs6000: Fix TARGET_POWERPC64 vs. TARGET_64BIT confusion

I gave Ke Wen bad advice, luckily David corrected me: it is true that we
cannot use TARGET_POWERPC64 on many 32-bit OSes, since either the kernel
or userland does not save the top half of the 64-bit integer registers,
but we do not have to care about that in separate patterns or related
code.  The flag is automatically not enabled by default on targets that
do not handle this correctly.

This patch fixes it.

Segher

2020-11-06  Segher Boessenkool  <segher@kernel.crashing.org>

	PR target/96933
	* config/rs6000/rs6000.c (rs6000_expand_vector_init): Use
	TARGET_POWERPC64 instead of TARGET_64BIT.