Place static closures in their own section.

Summary:
The primary reason for doing this is assisting debuggability:
if static closures are all in the same section, they are
guaranteed to be adjacent to one another.  This will help
later when we add some code that takes section start/end and
uses this to sanity-check the sections.

Part of remove HEAP_ALLOCED patch set (#8199)

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>

Test Plan: validate

Reviewers: simonmar, austin

Subscribers: simonmar, ezyang, carter, thomie

Differential Revision: https://phabricator.haskell.org/D263

GHC Trac Issues: #8199