Normalise benchmarks

This attempts to normalize the benchmark layout a bit to make it easier to use run them in one-off settings. Namely:

• Prefer to set PROG_ARGS rather than SRC_RUNTEST_OPTS for arguments destined for the program
• Use Main.l?hs as the main module name if possible.
• Eliminate redundant -cpp flags
• Use LANGUAGE pragmas instead of compiler flags
• Use OPTION_GHC pragmas instead of compiler flags where possible
• Rewrite tabs to spaces to avoid warnings
• Move -package flags to a new SRC_DEPS variable, where it can be better managed by the build system (although such support will only come in !24 (merged))