linux-phytec-fsl

selftests/powerpc: Specify we expect to build with std=gnu99

We have some tests that assume we're using std=gnu99, which is fine on
most compilers, but some old compilers use a different default.

So make it explicit that we want to use std=gnu99.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>