selftests/powerpc: Fix Makefiles for headers_install change

Commit b2d35fa5 ("selftests: add headers_install to lib.mk")
introduced a requirement that Makefiles more than one level below the
selftests directory need to define top_srcdir, but it didn't update
any of the powerpc Makefiles.

This broke building all the powerpc selftests with eg:

  make[1]: Entering directory '/src/linux/tools/testing/selftests/powerpc'
  BUILD_TARGET=/src/linux/tools/testing/selftests/powerpc/alignment; mkdir -p $BUILD_TARGET; make OUTPUT=$BUILD_TARGET -k -C alignment all
  make[2]: Entering directory '/src/linux/tools/testing/selftests/powerpc/alignment'
  ../../lib.mk:20: ../../../../scripts/subarch.include: No such file or directory
  make[2]: *** No rule to make target '../../../../scripts/subarch.include'.
  make[2]: Failed to remake makefile '../../../../scripts/subarch.include'.
  Makefile:38: recipe for target 'alignment' failed

Fix it by setting top_srcdir in the affected Makefiles.

Fixes: b2d35fa5 ("selftests: add headers_install to lib.mk")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>