powerpc/selftests/copyloops: extend selftest to exercise __copy_tofrom_user_power7_vmx

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 <sayalip@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260304122201.153049-2-sayalip@linux.ibm.com