Cygwin and Mingw floating-point fixes.

Deal with silent-underflow errors in float4 for cygwin and mingw by
using our strtof() wrapper; deal with misrounding errors by adding
them to the resultmap. Some slight reorganization of declarations was
done to avoid duplicating material between cygwin.h and win32_port.h.

While here, remove from the resultmap all references to
float8-small-is-zero; inspection of cygwin output suggests it's no
longer required there, and the freebsd/netbsd/openbsd entries should
no longer be necessary (these date back to c. 2000). This commit
doesn't remove the file itself nor the documentation references for
it; that will happen in a subsequent commit if all goes well.

src/include/port/win32_port.h

@@ -510,16 +510,23 @@ typedef unsigned short mode_t;
 #define isnan(x) _isnan(x)
 #endif
 
-#if defined(_MSC_VER) && (_MSC_VER < 1900)
+#if (defined(_MSC_VER) && (_MSC_VER < 1900)) || \
+	defined(__MINGW32__) || defined(__MINGW64__)
 /*
  * VS2013 has a strtof() that seems to give correct answers for valid input,
  * even on the rounding edge cases, but which doesn't handle out-of-range
  * input correctly. Work around that.
+ *
+ * Mingw claims to have a strtof, and my reading of its source code suggests
+ * that it ought to work (and not need this hack), but the regression test
+ * results disagree with me; whether this is a version issue or not is not
+ * clear. However, using our wrapper (and the misrounded-input variant file,
+ * already required for supporting ancient systems) can't make things any
+ * worse, except for a tiny performance loss when reading zeros.
+ *
+ * See also cygwin.h for another instance of this.
  */
-#define HAVE_BUGGY_WINDOWS_STRTOF 1
-extern float pg_strtof(const char *nptr, char **endptr);
-#define strtof(a,b) (pg_strtof((a),(b)))
-
+#define HAVE_BUGGY_STRTOF 1
 #endif
 
 /* Pulled from Makefile.port in MinGW */