selftests/powerpc: Fix standalone powerpc build

The changes to enable building with a separate output directory, in
commit a8ba798b ("selftests: enable O and KBUILD_OUTPUT") broke
building the powerpc selftests on their own, eg:

 $ cd tools/testing/selftests/powerpc; make

It was partially fixed in commit e53aff45 ("selftests: lib.mk Fix
individual test builds"), which defined OUTPUT for standalone tests. But
that only defines OUTPUT within the Makefile, the value is not exported
so sub-shells can't see it. We could export OUTPUT, but it's actually
cleaner to just expand the value of OUTPUT before we invoke the shell.

Fixes: a8ba798b ("selftests: enable O and KBUILD_OUTPUT")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>