call popcount32/64 directly on non-x86 platforms

Currently, all platforms must indirect through a function pointer to call
popcount on a word-sized input, even though we don't arrange for a fast
implementation on non-x86 to make it worthwhile.

0001 moves some declarations around so that "slow" popcount functions are
called directly on non-x86 platforms.

0002 was an idea to simplify and unify the coding for the slow functions.

Also attached is a test module for building microbenchmarks.

On a Power8 machine using gcc 4.8, and running
time ./inst/bin/psql -c 'select drive_popcount(100000, 1024)'

I get

master: 647ms
0001: 183ms
0002: 228ms

So 0001 is a clear winner on that platform. 0002 is still good, but slower
than 0001 for some reason, and it turns out that on master, gcc does emit a
popcnt instruction from the intrinsic:

0000000000000000 <pg_popcount32_slow>:
0: f4 02 63 7c popcntw r3,r3
4: b4 07 63 7c extsw r3,r3
8: 20 00 80 4e blr
...

The gcc docs mention a flag for this, but I'm not sure why it seems not to
need it:

https://gcc.gnu.org/onlinedocs/gcc/RS_002f6000-and-PowerPC-Options.html#RS_002f6000-and-PowerPC-Options

Maybe that's because the machine I used was ppc64le, but I'm not sure a ppc
binary built like this is portable to other hardware. For that reason,
maybe 0002 is a good idea.

--
John Naylor
EDB: http://www.enterprisedb.com