powerpc/vdso: Improve linker flags

When clang's -Qunused-arguments is dropped from KBUILD_CPPFLAGS, there
are several warnings in the PowerPC vDSO:

  clang-16: error: -Wl,-soname=linux-vdso32.so.1: 'linker' input unused [-Werror,-Wunused-command-line-argument]
  clang-16: error: -Wl,--hash-style=both: 'linker' input unused [-Werror,-Wunused-command-line-argument]
  clang-16: error: argument unused during compilation: '-shared' [-Werror,-Wunused-command-line-argument]

  clang-16: error: argument unused during compilation: '-nostdinc' [-Werror,-Wunused-command-line-argument]
  clang-16: error: argument unused during compilation: '-Wa,-maltivec' [-Werror,-Wunused-command-line-argument]

The first group of warnings point out that linker flags were being added
to all invocations of $(CC), even though they will only be used during
the final vDSO link. Move those flags to ldflags-y.

The second group of warnings are compiler or assembler flags that will
be unused during linking. Filter them out from KBUILD_CFLAGS so that
they are not used during linking.

Additionally, '-z noexecstack' was added directly to the ld_and_check
rule in commit 1d53c019 ("powerpc/vdso: link with -z noexecstack")
but now that there is a common ldflags variable, it can be moved there.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>