compiler, runtime: remove hmap field from maptypes

This is the gofrontend version of https://golang.org/cl/91796.

This is part of that CL, just the compiler change and required runtime
changes, in preparation for updating libgo to 1.11.

Relevant part of original CL description:

    The hmap field in the maptype is only used by the runtime to check the sizes of
    the hmap structure created by the compiler and runtime agree.

    Comments are already present about the hmap structure definitions in the
    compiler and runtime needing to be in sync.

Change-Id: I5ffcb281211029ca94ef8739d126db886fa4f249
Reviewed-on: https://go-review.googlesource.com/130976
Reviewed-by: Than McIntosh <thanm@google.com>
4 files changed