git-annex: adjust assert for Ruby pre-1.8.7

Within a block, Ruby pre-1.8.7 gets confused about the parens in
this case.

Fixes mistydemeo/tigerbrew#194.
Ensure HOMEBREW_ARCHFLAGS is always a string
Move -m32 and -m64 filtering up one level

These flags cannot be used behind -Xarch_<arch>, so stop filtering them
there.
Allow disabling arch flag filtering
llvm: build universal under superenv