Proposal for enabling auto-vectorization for checksum calculations

Hello,
This patch enables more compiler autovectorization for the checksum
calculations.
This code is particularly well suited for autovectorization, so just
adding pg_attribute_target and some simple dynamic dispatch logic we
can get improved vectorization.
This gives about a 2x speedup in a synthetic benchmark for
pg_checksum, which is also included as a seperate patch file.

Additionally, another 2x performance increase in the synthetic
benchmark with AVX2 can be obtained if N_SUMS was changed to 64.
However, this would change the results of the checksum. This isn't
included in this patch, but I think it is worth considering for the
future

One additional factor, without explicitly passing some optimization
flag like -O2 the makefile build won't autovectorize any of the code.
However, the meson based build does this automatically.