hsc2hs's C compiler invocations are problematic

Currently hsc2hs invokes ld and cc itself, allowing the commands and arguments to be specified via its --cc, --cflag, --ld, and --ldflag options. This design is problematic since it introduces the possibility for inconsistency between the toolchain used by hsc2hs and that used by GHC itself. This has already bitten us in the case of Darwin (see !6236 (closed)) and will certainly bite us further once we have #19877 (closed).

Frankly, I don't see why hsc2hs should be calling cc at all; it seems like it would be generally easier if it rather simply used GHC to drive compilation and linking, giving us a single point of configuration.