powerpc/vdso: No need to undef powerpc for 64-bit build

The vdso Makefile adds -U$(ARCH) to CPPFLAGS for the vdso64.lds linker
script. ARCH is always powerpc, so it becomes -Upowerpc, which means
undefine the "powerpc" symbol.

But the 64-bit compiler doesn't define powerpc in the first place,
compare:

  $ gcc-5.1.0-nolibc/powerpc64-linux/bin/powerpc64-linux-gcc -m32 -E -dM - </dev/null | grep -w powerpc
  #define powerpc 1
  $ gcc-5.1.0-nolibc/powerpc64-linux/bin/powerpc64-linux-gcc -m64 -E -dM - </dev/null | grep -w powerpc
  $

So there's no need to undefine it for the 64-bit linker script.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231206115548.1466874-2-mpe@ellerman.id.au