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 a28e4b6 ("drm/amd/display: use
ARCH_HAS_KERNEL_FPU_SUPPORT").

Fixes: 01db473 ("powerpc: implement ARCH_HAS_KERNEL_FPU_SUPPORT")
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Reported-by: Guenter Roeck <[email protected]>
Closes: https://lore.kernel.org/lkml/[email protected]/
Signed-off-by: Samuel Holland <[email protected]>
Tested-by: Guenter Roeck <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://msgid.link/[email protected]

Author: SiFiveHolland

arch/powerpc/Kconfig

@@ -137,7 +137,7 @@ config PPC
 	select ARCH_HAS_GCOV_PROFILE_ALL
 	select ARCH_HAS_HUGEPD			if HUGETLB_PAGE
 	select ARCH_HAS_KCOV
-	select ARCH_HAS_KERNEL_FPU_SUPPORT	if PPC_FPU
+	select ARCH_HAS_KERNEL_FPU_SUPPORT	if PPC64 && PPC_FPU
 	select ARCH_HAS_MEMBARRIER_CALLBACKS
 	select ARCH_HAS_MEMBARRIER_SYNC_CORE
 	select ARCH_HAS_MEMREMAP_COMPAT_ALIGN	if PPC_64S_HASH_MMU