rts: Query system rlimit for maximum address-space size
rts: Query system rlimit for maximum address-space size

When we attempt to reserve the heap, we query the system's rlimit to
establish the starting point for our search over sizes.

Test Plan: Validate

Reviewers: erikd, simonmar

Reviewed By: simonmar

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14492

Differential Revision: https://phabricator.haskell.org/D4754

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc