x86: Add avx2 optimized functions for the wchar_t strcpy family

x86: Add avx2 optimized functions for the wchar_t strcpy family

Implemented:
    wcscat-avx2  (+ 744 bytes
    wcscpy-avx2  (+ 539 bytes)
    wcpcpy-avx2  (+ 577 bytes)
    wcsncpy-avx2 (+1108 bytes)
    wcpncpy-avx2 (+1214 bytes)
    wcsncat-avx2 (+1085 bytes)

Performance Changes:
    Times are from N = 10 runs of the benchmark suite and are reported
    as geometric mean of all ratios of New Implementation / Best Old
    Implementation. Best Old Implementation was determined with the
    highest ISA implementation.

    wcscat-avx2     -> 0.975
    wcscpy-avx2     -> 0.591
    wcpcpy-avx2     -> 0.698
    wcsncpy-avx2    -> 0.730
    wcpncpy-avx2    -> 0.711
    wcsncat-avx2    -> 0.954

Code Size Changes:
    This change  increase the size of libc.so by ~5.5kb bytes. For
    reference the patch optimizing the normal strcpy family functions
    decreases libc.so by ~5.2kb.

Full check passes on x86-64 and build succeeds for all ISA levels w/
and w/o multiarch.