powerpc: Limit ARCH_HAS_KERNEL_FPU_SUPPORT to PPC64

powerpc: Limit ARCH_HAS_KERNEL_FPU_SUPPORT to PPC64

When building a 32-bit kernel, some toolchains do not allow mixing soft
float and hard float object files:

    LD      vmlinux.o
  powerpc64le-unknown-linux-musl-ld: lib/test_fpu_impl.o uses hard float, arch/powerpc/kernel/udbg.o uses soft float
  powerpc64le-unknown-linux-musl-ld: failed to merge target specific data of file lib/test_fpu_impl.o
  make[2]: *** [scripts/Makefile.vmlinux_o:62: vmlinux.o] Error 1
  make[1]: *** [Makefile:1152: vmlinux_o] Error 2
  make: *** [Makefile:240: __sub-make] Error 2

This is not an issue when building a 64-bit kernel. To unbreak the
build, limit ARCH_HAS_KERNEL_FPU_SUPPORT to 64-bit kernels. This is okay
because the only real user of this option, amdgpu, was previously
limited to PPC64 anyway; see commit a28e4b672f04 ("drm/amd/display: use
ARCH_HAS_KERNEL_FPU_SUPPORT").

Fixes: 01db473e1aa3 ("powerpc: implement ARCH_HAS_KERNEL_FPU_SUPPORT")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202405250851.Z4daYSWG-lkp@intel.com/
Reported-by: Guenter Roeck <linux@roeck-us.net>
Closes: https://lore.kernel.org/lkml/eeffaec3-df63-4e55-ab7a-064a65c00efa@roeck-us.net/
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240529162852.1209-1-samuel.holland@sifive.com