rts/linker/{SymbolExtras,elf_got}.c: map code as read-only

protect mmaped addresses from writes after being initially manipulated

Test Plan: ./validate

Reviewers: bgamari, erikd, simonmar

Reviewed By: bgamari

Subscribers: angerman, carlostome, rwbarton, thomie, carter

GHC Trac Issues: #14069

Differential Revision: https://phabricator.haskell.org/D4817