Make LLVM output robust to -dead_strip on mach-o platforms

This reverses commit 1686f30951292e94bf3076ce8b3eafb0bcbba91d (Mangle
.subsections_via_symbols away., D3287), and implements proper support
for `-dead_strip` via the injection of `.alt_entry` symbols for the
function definition pointing to the beginning of the prefix data.

This is the result of a lengthy discussion with rwbarton, and the
following llvm-dev mailing list thread:
http://lists.llvm.org/pipermail/llvm-dev/2017-March/110733.html

The essential problem is that there is no reference from a function to
its info table.  This combined with `.subsections_via_symbols`, which
llvm emits unconditionally, leads the linker to believe that the prefix
data is unnecessary and stripping it away if presented with the
`-dead_strip` flag.

The NCG has for this purpose special $dsp (dead strip preventer) symbols
and adds a relocation to the end of each function body pointing to that
function's $dsp symbol. We cannot easily do the same thing via LLVM.
Instead we use the `.alt_entry` directive on the function symbol, which
causes the linker to treat it as a continuation of the previous symbol,
namely the $dsp symbol. As a result the function body will not be
separated from its info table.

Reviewers: erikd, austin, rwbarton, bgamari

Reviewed By: bgamari

Subscribers: michalt, thomie

Differential Revision: https://phabricator.haskell.org/D3290

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