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ARROW-16850: [C++] Copy CSV data field and end chars separately (#13394)

It's a finding when evaluating Arm Statistical Profiling Extension(SPE)
on Neoverse-N1 with Arrow CSV writer benchmark. With SPE, we are able to
locate precisely the machine code causing heavy cache miss or the exact
branch suffering from high misprediction rate.

CSV writer benchmark reveals relatively high branch misprediction rate
inside glibc `memcpy` function. The branch is comparing the buffer size
against 8, and to run different copying code per comparison result.

Arrow CSV writer populates one column at once. In the inner loop, it
does two `memcpy`. First one is to copy the field value(string) from
Arrow string array. Then it does another copy to append the delimiter
or end-of-line(if last column), which is at most 2 chars. Data in same
column is normally of similar size, but as we are copying a very short
delimiter after each data field, looks it makes CPU harder to predict
the correct code path in `memcpy` as input data size varies quickly.

This PR adds a trivial `CopyEndChars` routine to copy only the
delimiter and end-of-line. It improves performance significantly on
Arm Neoverse-N1. Skylake also sees mild improvement.

Lead-authored-by: Yibo Cai <yibo.cai@arm.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Antoine Pitrou <antoine@python.org>