Refer to 'mask' instead of 'block' in Control.Exception

Summary: More thorough version of a75383cdd46f7bb593639bc6d1628b068b78262a

Test Plan:
change of comments only

[skip ci]

Reviewers: austin, simonmar, ekmett

Reviewed By: austin, ekmett

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D239
Delete hack that was once needed to fix the build

Summary:
Introduced in 6c7b41cc2b24f533697a62bf1843507ae043fc97.

I checked the rest of that commit, and this is all that was left to revert.

Test Plan: x

Reviewers: ezyang, austin

Reviewed By: ezyang, austin

Subscribers: simonmar, ezyang, carter, thomie

Differential Revision: https://phabricator.haskell.org/D241
User's Guide: various unfolding-related fixes

Test Plan: harbormaster

Reviewers: austin

Reviewed By: austin

Subscribers: simonmar, ezyang, carter, thomie

Differential Revision: https://phabricator.haskell.org/D243
Fixes cyclic import on OS X(#9635)

Summary: Signed-off-by: Dave Laing <dave.laing.80@gmail.com>

Test Plan: Locally tested

Reviewers: thomie, austin

Reviewed By: thomie, austin

Subscribers: simonmar, ezyang, carter, thomie

Differential Revision: https://phabricator.haskell.org/D244

GHC Trac Issues: #9635