ghc: fix build on 10.9; cleanup

This patch fixes ```ghc 7.6.3``` compilation on OS X 10.9 DP5. See issue #20546.

Notes
=====

1) have compiled some large projects (Agda); seems to work okay
2) not yet tested on < 10.9 but should be okay given the changes are conditional
3) had to depend on ```apple_gcc42``` since ```fails_with``` logic alone did not seem to work (see comments)
4) per (3), someone who better understands homebrew's compiler selection logic will have to take a look
5) the RTS DTrace functionality may need additional testing
6) updated GHC bootstrap binary from ```ghc 7.4.2``` to ```ghc 7.6.3``` (they both run on baseline OS X 10.7)
7) removed -j1 option; seems no longer needed