powerpc/book3s64: Always build for power4 or later

When we're not compiling for a specific CPU, ie. none of the
CONFIG_POWERx_CPU options are set, and CONFIG_GENERIC_CPU *is* set, we
currently don't pass any -mcpu option to the compiler. This means the
compiler builds for a "generic" Power CPU.

But back in 2014 we dropped support for pre power4 CPUs in commit
468a3302

 ("powerpc: Drop support for pre-POWER4 cpus").

Given that, there's no point in building the kernel to run on pre power4
cpus. So update the flags we pass to the compiler when
CONFIG_GENERIC_CPU is set, to specify -mcpu=power4.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>