[Toybox] macOS sed

[enh]

according to https://stackoverflow.com/questions/12272065/sed-undefined-label-on-macos
macOS' sed only lets you branch backwards. this breaks mkflagsh and
getglobals in scripts/make.sh.

i'm assuming the answer is "you'll need `brew install gnu-sed` to
bootstrap, and can use a toybox sed from then on", but should we give
a clearer error? something like "if you're on macOS but i can't find
gsed, suggest `brew install`"?