SRT offset optimisation breaks on MachO platforms

2b0918c9 reduces the overhead of SRT references by encoding them as an offset on amd64, as such references are always possible to represent in a half-word in the small memory model.

Unfortunately, this appears to break on MachO platforms (e.g. Darwin) due to an object format limitation. Specifically, linking fails with:

/var/folders/3y/qjvrbynj45j_z4jt_l1k1qg00000gv/T/ghc32020_0/ghc_3.s:219:8: error:
     error: unsupported relocation with subtraction expression, symbol '_integerzmgmp_GHCziIntegerziType_quotInteger_closure' can not be undefined in a subtraction expression
            .long   _integerzmgmp_GHCziIntegerziType_quotInteger_closure-(_cOA_info)+0
                    ^
    |
219 |         .long   _integerzmgmp_GHCziIntegerziType_quotInteger_closure-(_cOA_info)+0
    |

Apparently MachO does not permit relocations' subtraction expressions to refer to undefined symbols. As far as I can tell this means that it is essentially impossible to express an offset between symbols living in different compilation units. This means that we lively can't use this optimisation on MachO platforms.

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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