Where possible generate calls to runtime makemap with int hint argument
during compile time instead of makemap with int64 hint argument.

This eliminates converting the hint argument for calls to makemap with
int64 hint argument for platforms where int64 values do not fit into
an argument of type int.

A similar optimization for makeslice was introduced in CL
golang.org/cl/27851.

386:
name         old time/op    new time/op    delta
NewEmptyMap    53.5ns ± 5%    41.9ns ± 5%  -21.56%  (p=0.000 n=10+10)
NewSmallMap     182ns ± 1%     165ns ± 1%   -8.92%  (p=0.000 n=10+10)

Change-Id: Ibd2b4c57b36f171b173bf7a0602b3a59771e6e44
Reviewed-on: https://go-review.googlesource.com/55142Reviewed-by: Keith Randall <khr@golang.org>