Remove dependence on -fwrapv semantics in a few places.

This commit attempts to update a few places, such as the money,
numeric, and timestamp types, to no longer rely on signed integer
wrapping for correctness.  This is intended to move us closer
towards removing -fwrapv, which may enable some compiler
optimizations.  However, there is presently no plan to actually
remove that compiler option in the near future.

Besides using some of the existing overflow-aware routines in
int.h, this commit introduces and makes use of some new ones.
Specifically, it adds functions that accept a signed integer and
return its absolute value as an unsigned integer with the same
width (e.g., pg_abs_s64()).  It also adds functions that accept an
unsigned integer, store the result of negating that integer in a
signed integer with the same width, and return whether the negation
overflowed (e.g., pg_neg_u64_overflow()).

Finally, this commit adds a couple of tests for timestamps near
POSTGRES_EPOCH_JDATE.

Author: Joseph Koshakow
Reviewed-by: Tom Lane, Heikki Linnakangas, Jian He
Discussion: https://postgr.es/m/CAAvxfHdBPOyEGS7s%2Bxf4iaW0-cgiq25jpYdWBqQqvLtLe_t6tw%40mail.gmail.com

src/backend/utils/adt/numeric.c

@@ -8113,7 +8113,7 @@ int64_to_numericvar(int64 val, NumericVar *var)
 	if (val < 0)
 	{
 		var->sign = NUMERIC_NEG;
-		uval = -val;
+		uval = pg_abs_s64(val);
 	}
 	else
 	{
@@ -11584,7 +11584,7 @@ power_var_int(const NumericVar *base, int exp, int exp_dscale,
 	 * Now we can proceed with the multiplications.
 	 */
 	neg = (exp < 0);
-	mask = abs(exp);
+	mask = pg_abs_s32(exp);
 
 	init_var(&base_prod);
 	set_var_from_var(base, &base_prod);