Cross-compile compiler-rt for macOS

Right now we only build clang for macOS cross-compilation but that's not enough anymore. We need to cross-compile compiler-rt as Mozilla does to deal with @available checks.

changed milestone to %Tor Browser: 10.0

added 1 deleted label

mentioned in issue #34229 (closed)

mentioned in issue #40020 (closed)

added Doing label

changed time estimate to 8h

mentioned in merge request !21 (closed)

That's been the PITA I expected it to be but I've something working, finally (+ an overall better understanding about what we are actually doing here )

added 14h of time spent at 2020-08-06

mentioned in merge request !23 (merged)

closed via merge request !23 (merged)

mentioned in commit c1bc5e42

mentioned in merge request !342 (closed)