Modified the SUFFIXES pseudo targets so that we remove all default suffixes

and explicitly declare all the ones we're using for LLVM.
This quickly cancels many of GNU Make's implicit rules and reduces build time.
The only caveat is that any new suffixes may need to be explictly added to
the .SUFFIXES pseudo target.

Removed the -only-static option as it is no longer used.

llvm-svn: 7979