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

arch/powerpc/kernel/vdso/Makefile

@@ -71,7 +71,7 @@ AS64FLAGS := -D__VDSO64__
 targets += vdso32.lds
 CPPFLAGS_vdso32.lds += -P -C -Upowerpc
 targets += vdso64.lds
-CPPFLAGS_vdso64.lds += -P -C -U$(ARCH)
+CPPFLAGS_vdso64.lds += -P -C
 
 # link rule for the .so file, .lds has to be first
 $(obj)/vdso32.so.dbg: $(src)/vdso32.lds $(obj-vdso32) $(obj)/vgettimeofday-32.o FORCE