[NFC][PowerPC] Update builtins-ppc-altivec.c to be run under `-faltivec-src-compat=mixed`

This patch adds the `-faltivec-src-compat=mixed` option to the
`builtins-ppc-altivec.c` test.

Currently, the default for `-faltivec-src-compat` is `mixed`. The reason we
explicitly specify `mixed` to the RUN lines of this test is because eventually,
the default will set to `xl`.

Having the default as `xl` changes the CHECKs of this test slightly, as it
reorders some of the `vector bool` and `vector pixel` CHECKs (since under the
`xl` option, `vector bool` and `vector pixel` are treated in the same way as
other vector scalars). Explicitly specifying `mixed` ensures that we are testing
pre-existing Clang behaviour.

Differential Revision: https://reviews.llvm.org/D106282