gn build: Change scudo's list of supported platforms to a whitelist.

Scudo only supports building for android/linux/fuchsia, so require target_os to
be one of linux/fuchsia to do a stage2_unix scudo build. Android is already
covered by the stage2_android* toolchains below.

Differential Revision: https://reviews.llvm.org/D71131

llvm/utils/gn/secondary/compiler-rt/lib/scudo/BUILD.gn

@@ -1,11 +1,7 @@
 import("//llvm/utils/gn/build/toolchain/compiler.gni")
 
 supported_toolchains = []
-# FIXME: On macOS, stage2_unix currently doesn't copy libc++ headers to
-# the out dir, but clang relies on them on mac to compile code that uses C++
-# standard library headers. scudo needs C++ standard library headers, so disable
-# this on mac until stage2_unix correctly copies libc++ headers.
-if (target_os != "win" && target_os != "mac")  {
+if (target_os == "linux" || target_os == "fuchsia")  {
   supported_toolchains += [ "//llvm/utils/gn/build/toolchain:stage2_unix" ]
 }
 if (android_ndk_path != "") {