sysroot.py: add support for non-darwin platforms

[thakis]

Summary

CMAKE_SYSROOT works fine here, and sysroot.py make-fake
borders on trivial here, but I suppose it's still nice
to have a consistent script to set these up across platforms.

And these are the platforms where we can do real sysroot management one
day.

[hans]

llvm/utils/sysroot.py

41

nice sysroot :)

llvm/utils/gn/build/BUILD.gn

if (is_clang) {
     if (use_relative_paths_in_debug_info) {
       cflags += [
         "-fdebug-compilation-dir",
         ".",
       ]
     }
   }
   if (sysroot != "") {
-    assert(current_os == "win", "FIXME: Make sysroot work on non-win")
+    assert(current_os != "mac" && current_os != "ios",
+           "FIXME: Make sysroot work on darwin")
+    if (current_os == "win") {
       assert(is_clang, "sysroot only works with clang-cl as host compiler")
       cflags += [ "/winsysroot" + rebase_path(sysroot, root_build_dir) ]
+    } else {
+      cflags += [ "--sysroot=" + rebase_path(sysroot, root_build_dir) ]
+    }
   }
 
   if (use_ubsan) {
     assert(is_clang && current_os == "linux",
            "ubsan only supported on Linux/Clang")
     cflags += [ "-fsanitize=undefined" ]
     ldflags += [ "-fsanitize=undefined" ]
   }

llvm/utils/sysroot.py

if sys.platform == 'win32':
 
         def mkjunction(dst, src):
             subprocess.check_call(['mklink', '/j', dst, src], shell=True)
         os.mkdir(out_dir)
         mkjunction(os.path.join(out_dir, 'VC'),
                    os.path.join(vsinstalldir, 'VC'))
         os.mkdir(os.path.join(out_dir, 'Windows Kits'))
         mkjunction(os.path.join(out_dir, 'Windows Kits', '10'), winsdk)
+    elif sys.platform == 'darwin':
+        assert False, "FIXME: Implement on darwin"
     else:
-        assert False, "FIXME: Implement on non-win"
+        os.symlink('/', out_dir)
 
     print('Done.')
+    abs_out_dir = os.path.abspath(out_dir)
     if sys.platform == 'win32':
         # CMake doesn't like backslashes in commandline args.
-        abs_out_dir = os.path.abspath(out_dir).replace(os.path.sep, '/')
+        abs_out_dir = abs_out_dir.replace(os.path.sep, '/')
         print('Pass -DLLVM_WINSYSROOT=' + abs_out_dir + ' to cmake.')
     else:
         print('Pass -DCMAKE_SYSROOT=' + abs_out_dir + ' to cmake.')
 
 
 def main():
     parser = argparse.ArgumentParser(description=__doc__)