powerpc/selftests/copyloops: extend selftest to exercise __copy_tofrom_user_power7_vmx

The new PowerPC VMX fast path (__copy_tofrom_user_power7_vmx) is not exercised by existing copyloops selftests. This patch updates the selftest to exercise the VMX variant, ensuring the VMX copy path is validated.

Changes include:
- COPY_LOOP=test___copy_tofrom_user_power7_vmx with -D VMX_TEST is used in existing selftest build targets.
- Inclusion of ../utils.c to provide get_auxv_entry() for hardware feature detection.
- At runtime, the test skips execution if Altivec is not available.
- Copy sizes above VMX_COPY_THRESHOLD are used to ensure the VMX path is taken.

This enables validation of the VMX fast path without affecting systems that do not support Altivec.

Signed-off-by Sayali Patil <[email protected]>

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