Use correct section types syntax for architecture

Previously GHC would always assume that section types began with `@` while
producing assembly, which is not true. For instance, in ARM assembly syntax
section types begin with `%`. This abstracts out section type pretty-printing
and adjusts it to correctly account for the target architectures assembly
flavor.

Reviewers: austin, hvr, Phyx

Reviewed By: Phyx

Subscribers: Phyx, rwbarton, thomie, erikd

GHC Trac Issues: #13937

Differential Revision: https://phabricator.haskell.org/D3712