Make use of compiler builtins and/or assembly for CLZ, CTZ, POPCNT.

Test for the compiler builtins __builtin_clz, __builtin_ctz, and
__builtin_popcount, and make use of these in preference to
handwritten C code if they're available.  Create src/port
infrastructure for "leftmost one", "rightmost one", and "popcount"
so as to centralize these decisions.

On x86_64, __builtin_popcount generally won't make use of the POPCNT
opcode because that's not universally supported yet.  Provide code
that checks CPUID and then calls POPCNT via asm() if available.
This requires indirecting through a function pointer, which is
an annoying amount of overhead for a one-instruction operation,
but it's probably not worth working harder than this for our
current use-cases.

I'm not sure we've found all the existing places that could profit
from this new infrastructure; but we at least touched all the
ones that used copied-and-pasted versions of the bitmapset.c code,
and got rid of multiple copies of the associated constant arrays.

While at it, replace c-compiler.m4's one-per-builtin-function
macros with a single one that can handle all the cases we need
to worry about so far.  Also, because I'm paranoid, make those
checks into AC_LINK checks rather than just AC_COMPILE; the
former coding failed to verify that libgcc has support for the
builtin, in cases where it's not inline code.

David Rowley, Thomas Munro, Alvaro Herrera, Tom Lane

Discussion: https://postgr.es/m/CAKJS1f9WTAGG1tPeJnD18hiQW5gAk59fQ6WK-vfdAKEHyRg2RA@mail.gmail.com

config/c-compiler.m4

@@ -273,60 +273,6 @@ AC_DEFINE(HAVE__BUILTIN_TYPES_COMPATIBLE_P, 1,
 fi])# PGAC_C_TYPES_COMPATIBLE
 
 
-# PGAC_C_BUILTIN_BSWAP16
-# -------------------------
-# Check if the C compiler understands __builtin_bswap16(),
-# and define HAVE__BUILTIN_BSWAP16 if so.
-AC_DEFUN([PGAC_C_BUILTIN_BSWAP16],
-[AC_CACHE_CHECK(for __builtin_bswap16, pgac_cv__builtin_bswap16,
-[AC_COMPILE_IFELSE([AC_LANG_SOURCE(
-[static unsigned long int x = __builtin_bswap16(0xaabb);]
-)],
-[pgac_cv__builtin_bswap16=yes],
-[pgac_cv__builtin_bswap16=no])])
-if test x"$pgac_cv__builtin_bswap16" = xyes ; then
-AC_DEFINE(HAVE__BUILTIN_BSWAP16, 1,
-          [Define to 1 if your compiler understands __builtin_bswap16.])
-fi])# PGAC_C_BUILTIN_BSWAP16
-
-
-
-# PGAC_C_BUILTIN_BSWAP32
-# -------------------------
-# Check if the C compiler understands __builtin_bswap32(),
-# and define HAVE__BUILTIN_BSWAP32 if so.
-AC_DEFUN([PGAC_C_BUILTIN_BSWAP32],
-[AC_CACHE_CHECK(for __builtin_bswap32, pgac_cv__builtin_bswap32,
-[AC_COMPILE_IFELSE([AC_LANG_SOURCE(
-[static unsigned long int x = __builtin_bswap32(0xaabbccdd);]
-)],
-[pgac_cv__builtin_bswap32=yes],
-[pgac_cv__builtin_bswap32=no])])
-if test x"$pgac_cv__builtin_bswap32" = xyes ; then
-AC_DEFINE(HAVE__BUILTIN_BSWAP32, 1,
-          [Define to 1 if your compiler understands __builtin_bswap32.])
-fi])# PGAC_C_BUILTIN_BSWAP32
-
-
-
-# PGAC_C_BUILTIN_BSWAP64
-# -------------------------
-# Check if the C compiler understands __builtin_bswap64(),
-# and define HAVE__BUILTIN_BSWAP64 if so.
-AC_DEFUN([PGAC_C_BUILTIN_BSWAP64],
-[AC_CACHE_CHECK(for __builtin_bswap64, pgac_cv__builtin_bswap64,
-[AC_COMPILE_IFELSE([AC_LANG_SOURCE(
-[static unsigned long int x = __builtin_bswap64(0xaabbccddeeff0011);]
-)],
-[pgac_cv__builtin_bswap64=yes],
-[pgac_cv__builtin_bswap64=no])])
-if test x"$pgac_cv__builtin_bswap64" = xyes ; then
-AC_DEFINE(HAVE__BUILTIN_BSWAP64, 1,
-          [Define to 1 if your compiler understands __builtin_bswap64.])
-fi])# PGAC_C_BUILTIN_BSWAP64
-
-
-
 # PGAC_C_BUILTIN_CONSTANT_P
 # -------------------------
 # Check if the C compiler understands __builtin_constant_p(),
@@ -423,6 +369,33 @@ fi])# PGAC_C_COMPUTED_GOTO
 
 
 
+# PGAC_CHECK_BUILTIN_FUNC
+# -----------------------
+# This is similar to AC_CHECK_FUNCS(), except that it will work for compiler
+# builtin functions, as that usually fails to.
+# The first argument is the function name, eg [__builtin_clzl], and the
+# second is its argument list, eg [unsigned long x].  The current coding
+# works only for a single argument named x; we might generalize that later.
+# It's assumed that the function's result type is coercible to int.
+# On success, we define "HAVEfuncname" (there's usually more than enough
+# underscores already, so we don't add another one).
+AC_DEFUN([PGAC_CHECK_BUILTIN_FUNC],
+[AC_CACHE_CHECK(for $1, pgac_cv$1,
+[AC_LINK_IFELSE([AC_LANG_PROGRAM([
+int
+call$1($2)
+{
+    return $1(x);
+}], [])],
+[pgac_cv$1=yes],
+[pgac_cv$1=no])])
+if test x"${pgac_cv$1}" = xyes ; then
+AC_DEFINE_UNQUOTED(AS_TR_CPP([HAVE$1]), 1,
+                   [Define to 1 if your compiler understands $1.])
+fi])# PGAC_CHECK_BUILTIN_FUNC
+
+
+
 # PGAC_PROG_VARCC_VARFLAGS_OPT
 # -----------------------
 # Given a compiler, variable name and a string, check if the compiler