Fix busy-wait in SysTools.builderMainLoop

Test T13701 was failing sporadically. The problem manifested while the
test was run on a system under load. Profiling showed the increased
allocations were in SysTools.builderMainLoop.loop, during calls to the
assembler. This was due to loop effectively busy-waiting from when both
stdin and stderr handles were closed, until getProcessExitCode
succeeded.

This is fixed by removing exit code handling from loop. We now wait for
loop to finish, then read the exit code with waitForProcess.

Some exception safety is added: the readerProc threads will now be
killed and the handles will be closed if an exception is thrown.

A TODO saying that threads dying is not accounted for is removed. I
believe that this case is handled by readerProc sending EOF in a finally
clause.

Test Plan:
Replicate test failures using procedure on the ticket, verify that they
do not occur with this patch.

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #13987

Differential Revision: https://phabricator.haskell.org/D3748