powerpc/vdso: Drop -DCC_USING_PATCHABLE_FUNCTION_ENTRY from 32-bit flags with clang

powerpc/vdso: Drop -DCC_USING_PATCHABLE_FUNCTION_ENTRY from 32-bit flags with clang

After commit 73cdf24e81e4 ("powerpc64: make clang cross-build
friendly"), building 64-bit little endian + CONFIG_COMPAT=y with clang
results in many warnings along the lines of:

  $ cat arch/powerpc/configs/compat.config
  CONFIG_COMPAT=y

  $ make -skj"$(nproc)" ARCH=powerpc LLVM=1 ppc64le_defconfig compat.config arch/powerpc/kernel/vdso/
  ...
  In file included from <built-in>:4:
  In file included from lib/vdso/gettimeofday.c:6:
  In file included from include/vdso/datapage.h:15:
  In file included from include/vdso/cache.h:5:
  arch/powerpc/include/asm/cache.h:77:8: warning: unknown attribute 'patchable_function_entry' ignored [-Wunknown-attributes]
     77 | static inline u32 l1_icache_bytes(void)
        |        ^~~~~~
  include/linux/compiler_types.h:235:58: note: expanded from macro 'inline'
    235 | #define inline inline __gnu_inline __inline_maybe_unused notrace
        |                                                          ^~~~~~~
  include/linux/compiler_types.h:215:34: note: expanded from macro 'notrace'
    215 | #define notrace                 __attribute__((patchable_function_entry(0, 0)))
        |                                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ...

arch/powerpc/Makefile adds -DCC_USING_PATCHABLE_FUNCTION_ENTRY to
KBUILD_CPPFLAGS, which is inherited by the 32-bit vDSO. However, the
32-bit little endian target does not support
'-fpatchable-function-entry', resulting in the warnings above.

Remove -DCC_USING_PATCHABLE_FUNCTION_ENTRY from the 32-bit vDSO flags
when building with clang to avoid the warnings.

Fixes: 73cdf24e81e4 ("powerpc64: make clang cross-build friendly")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260311-ppc-vdso-drop-cc-using-pfe-define-clang-v1-1-66c790e22650@kernel.org