add emscripten to std.build.Target.defaultAbi

After upgrading to LLVM 9 we now have the emscripten OS available. Fix
the compile error in this function by specifying the default ABI for
emscripten.
23f339f1b4Andrew Kelley committed on 9/22/2019, 11:10:18 PMparentd6f7b57
1 file changedLine totals unavailable