rename platform=darwin -> platform={ios,macosx}, fix ios simulator build
update README.md with darwin->{macosx,ios} rename
Merge pull request #281 from joshuawarner32/darwin-platform-reorg

rename platform=darwin -> platform={ios,macosx}, fix ios simulator build
allow stress and stress-major as valid mode= settings in the makefile (note: all tests fail in both modes)