Summary:
This reverts commit 4748f593. The fix for #9423
was reverted because this commit introduced a C function setIOManagerControlFd()
(defined in Schedule.c) defined for all OS types, while the prototype
(in includes/rts/IOManager.h) was only included when mingw32_HOST_OS is
not defined. This broke Windows builds.

This commit reverts the original commit and resolves the problem by only defining
setIOManagerControlFd() when mingw32_HOST_OS is defined. Hence the missing prototype
error should not occur on Windows.

In addition, since the io_manager_control_wr_fd field of the Capability struct is only
usd by the setIOManagerControlFd, this commit includes the io_manager_control_wr_fd
field in the Capability struct only when mingw32_HOST_OS is not defined.

Test Plan: Try to compile successfully on all platforms.

Reviewers: austin

Reviewed By: austin

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D174