Keep C main separate from rts lib and link it in for standalone progs
Previously the object code for the C main function lived in the rts lib, however this is a problem when the rts is built as a shared lib. With Windows DLLs it always causes problems while on ELF systems it's a problem when the user decides to use their own C main function rather than a Haskell Main.main. So instead we now put main in it's own tiny little static lib libHSrtsmain.a which we install next to the rts libs. Whenever ghc links a program (without -no-hs-main) then it also links in -lHSrtsmain. For consistency we always do it this way now rather than trying to do it differently for static vs shared libraries.