x86[_64] memset and rep stos

[Rich Felker <dalias@libc.org>]

Doing some timings on the new proposed memset code, I found it was
pathologically slow on my Atom D510 (32-bit) when reaching sizes
around 2k - 16k. Like 4x slower than the old code. Apparently the
issue is that the work being done to align the destination mod 4
misaligns it mod higher powers of two, and "rep stos" performs
pathologically bad when it's not cache-line-aligned, or something like
that. On my faster 64-bit system alignment mod 16 also seems to make a
difference, but less - it's 1.5x slower misaligned mod 16.

I also found that on the 32-bit Atom, there seems to be a huge jump in
speed at size 1024 -- sizes just below 1024 are roughly 2x slower.
Since it otherwise doesn't make a measurable difference, it seems
preferable _not_ to try to reduce the length of the rep stos to avoid
writing the same bytes multiple times but simply use the max allowable
length.

Combined with the first issue, it seems we should "round up to a
multiple of 16" rather than "add 16 then round down to a multiple of
16". Not only does this avoid reducing the length of the rep stos; it
also preserves any higher-than-16 alignment that might be preexisting,
in case even higher alignments are faster.

Rich