the optimizer was deleting compiler_rt symbols, so I changed
the linkage type from LinkOnce to Weak
also changed LinkOnce to mean linkonce_odr in llvm and
Weak to mean weak_odr in llvm.
See #563
* try some macos travis stuff
* put c in the link libs for macos since we always link with libSystem
* for non-native targets on macos, allow runtime symbol resolution
- it's causing an infinite loop in LLD.
* for macos, always build compiler_rt and turn on LinkOnce because
compiler_rt on darwin is missing some stuff.