To cope with the changes in:

  12c89130 ("x86/asm/memcpy_mcsafe: Add write-protection-fault handling")
  60622d68 ("x86/asm/memcpy_mcsafe: Return bytes remaining")
  bd131544 ("x86/asm/memcpy_mcsafe: Add labels for __memcpy_mcsafe() write fault handling")
  da7bc9c5 ("x86/asm/memcpy_mcsafe: Remove loop unrolling")

This needed introducing a file with a copy of the mcsafe_handle_tail()
function, that is used in the new memcpy_64.S file, as well as a dummy
mcsafe_test.h header.

Testing it:

  $ nm ~/bin/perf | grep mcsafe
  0000000000484130 T mcsafe_handle_tail
  0000000000484300 T __memcpy_mcsafe
  $
  $ perf bench mem memcpy
  # Running 'mem/memcpy' benchmark:
  # function 'default' (Default memcpy() provided by glibc)
  # Copying 1MB bytes ...

      44.389205 GB/sec
  # function 'x86-64-unrolled' (unrolled memcpy() in arch/x86/lib/memcpy_64.S)
  # Copying 1MB bytes ...

      22.710756 GB/sec
  # function 'x86-64-movsq' (movsq-based memcpy() in arch/x86/lib/memcpy_64.S)
  # Copying 1MB bytes ...

      42.459239 GB/sec
  # function 'x86-64-movsb' (movsb-based memcpy() in arch/x86/lib/memcpy_64.S)
  # Copying 1MB bytes ...

      42.459239 GB/sec
  $

This silences this perf tools build warning:

  Warning: Kernel ABI header at 'tools/arch/x86/lib/memcpy_64.S' differs from latest version at 'arch/x86/lib/memcpy_64.S'

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mika Penttilä <mika.penttila@nextfour.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-igdpciheradk3gb3qqal52d0@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>