Dynamically built GHC crashes on MacOS

Since 79d5427e (which add support for dynamically linking GHC), the default ./hadrian/build.sh is broken for MacOS. In particular, the ghc built always crashes with a "Symbol not found" error. Example:

$ ./hadrian/build.sh -j -c 
# << fails shortly after building first ghc >>
$ _build/stage1/bin/ghc
dyld: Symbol not found: _ffi_type_double
  Referenced from: _build/stage1/lib/../lib/x86_64-osx-ghc-8.7.20181202/libHSghci-8.7-ghc8.7.20181202.dylib
  Expected in: flat namespace
 in _build/stage1/lib/../lib/x86_64-osx-ghc-8.7.20181202/libHSghci-8.7-ghc8.7.20181202.dylib
[1]    52701 abort      _build/stage1/bin/ghc

See https://phabricator.haskell.org/rGHC79d5427e1f9de02c0b171bf5db46b6b49c6f85e3 for more discussion. The core of the issue seems to be that -Wl,-dead_strip_dylibs accidentally strips some essential libraries (libffi is one of these).

Trac metadata

Trac field	Value
Version	8.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Build System (Hadrian)
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