libc: ARM64: fix memset for non-standard ZVA sizes

372f19e9 ("libc: ARM64: update memset/strlen/memcpy/memmove to
newlib/cortex-strings") introduced a bug in memset, only occurring
on the [set_long + zero + non-standard ZVA size] path, more
specifically when DCZID_EL0 reports a size different to 64 or 128.

On platforms with such sizes reported by DCZID_EL0, various string*
unit tests fail due to memset zeroing memory before and/or after the
area it is supposed to set.

Test: bionic-unit-tests --gtest_filter=string*
Change-Id: Idb80c0269226e40e343645a58608e3f324378468