Link rts symbols weakly

Motivation

Currently rts symbols are undefined in ghc shared libraries (on Linux).

This causes:

$ ldd -r libHSzlib-0.6.2-BdOecmQ0rL1HOiGypdSgxY-ghc8.6.5.so | grep "undefined symbol" | wc -l
459
$ ldd -r libHSzlib-0.6.2-BdOecmQ0rL1HOiGypdSgxY-ghc8.6.5.so | grep "undefined symbol"
undefined symbol: stg_gc_unpt_r1	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_setThreadAllocationCounterzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_getThreadAllocationCounterzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_traceMarkerzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_traceEventzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_clearCCSzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_getApStackValzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_unpackClosurezh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_newBCOzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_mkApUpd0zh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_numSparkszh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_getSparkzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_compactSizzezh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_compactAddWithSharingzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_compactAddzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_compactFixupPointerszh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_compactAllocateBlockzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_compactGetNextBlockzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_compactGetFirstBlockzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_compactContainsAnyzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_compactContainszh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_compactResizzezh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_compactNewzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_makeStableNamezh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_deRefStablePtrzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_makeStablePtrzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_finalizzeWeakzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_deRefWeakzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_addCFinalizzerToWeakzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_mkWeakNoFinalizzerzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_mkWeakzh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
undefined symbol: stg_threadStatuszh	(/usr/lib64/libHSghc-prim-0.5.3-ghc8.6.5.so)
:
:

Apparently this is due to the RTS being chosen at runtime.

$ objdump -x libHSzlib-0.6.2-BdOecmQ0rL1HOiGypdSgxY-ghc8.6.5.so | grep NEEDED | grep libHSrts
$ 

This probably affects all versions of ghc with shared libraries.

Proposal

Linking libHSrts symbols as weak symbols should prevent these undefined symbol warnings.