This patch merges the PostgreSQL 9.5 implementation of SSE4.2 into GPDB.
The SSE4.2 implementation was lifted right out of PostgreSQL without
change to make merging later PostgreSQL releases easier.

Update win32 configuration to support SSE4.2 runtime checks

This change was pulled from "src/include/pg_config.h.win32" from the
commits below.

configure.in changes, determining if cpu instruction for perfomring runtime
checks are availible, and moving some of the code to port SSE4.2 came from the
following PostgreSQL commits:

commit 3dc2d62d
Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date:   Tue Apr 14 17:05:03 2015 +0300

    Use Intel SSE 4.2 CRC instructions where available.

    Modern x86 and x86-64 processors with SSE 4.2 support have special
    instructions, crc32b and crc32q, for calculating CRC-32C. They greatly
    speed up CRC calculation.

    Whether the instructions can be used or not depends on the compiler and the
    target architecture. If generation of SSE 4.2 instructions is allowed for
    the target (-msse4.2 flag on gcc and clang), use them. If they are not
    allowed by default, but the compiler supports the -msse4.2 flag to enable
    them, compile just the CRC-32C function with -msse4.2 flag, and check at
    runtime whether the processor we're running on supports it. If it doesn't,
    fall back to the slicing-by-8 algorithm. (With the common defaults on
    current operating systems, the runtime-check variant is what you get in
    practice.)

    Abhijit Menon-Sen, heavily modified by me, reviewed by Andres Freund.

commit b4eb2d16
Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date:   Tue Apr 14 19:56:00 2015 +0300
	On gcc and clang, the _mm_crc32_u8 and _mm_crc32_u64 intrinsics are not
	defined at all, when not building with -msse4.2. But on icc, they are.
	So we cannot assume that if those intrinsics are defined, we can always use
	them safely, we might still need the runtime check.

	To fix, check if the __SSE_4_2__ preprocessor symbol is defined. That's
	supposed to be defined only when the compiler is targeting a processor that
	has SSE 4.2 support.

	Per buildfarm members fulmar and okapi.