The rdynamic tests and feature are marked broken on windows.
This is because the flag used doesn't exist and the symbol lookup
in the test did not account for platform differences in name mangling.

This commit fixes the flag and tests for rdynamic on windows.

Test Plan:
make TEST="rdynamic"

on both x86 and x86_64

Reviewers: austin, thomie, bgamari

Reviewed By: thomie, bgamari

Subscribers: #ghc_windows_task_force

Differential Revision: https://phabricator.haskell.org/D1149

GHC Trac Issues: #9381