powerpc/64s: Fix GENERIC_CPU build flags for PPC970 / G5

powerpc/64s: Fix GENERIC_CPU build flags for PPC970 / G5

[ Upstream commit 26cdaee976981233a5bd76fda2ba1ca1ce15b142 ]

Big-endian GENERIC_CPU supports 970, but builds with -mcpu=power5.
POWER5 is ISA v2.02 whereas 970 is v2.01 plus Altivec. 2.02 added
the popcntb instruction which a compiler might use.

Use -mcpu=power4.

Fixes: d00b0f675bda ("powerpc/64s: Remove POWER4 support")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220921014103.587954-1-npiggin@gmail.com
Signed-off-by: Sasha Levin <sashal@kernel.org>