Fix ubuntu build on clang 10

[Fasjeit]

/home/appveyor/projects/corefx/src/Native/Unix/System.Native/pal_io.c(917,64): error G8A08D350: use of function '__builtin_alloca' is discouraged; there is no way to check for failure but failure may still occur, resulting in a possibly exploitable security vulnerability [-Werror,-Walloca] [/home/appveyor/projects/corefx/src/Native/build-native.proj]
      struct pollfd* pollfds = (struct pollfd*)(useStackBuffer ? alloca(bufferSize) : malloc(bufferSize));
                                                                 ^
  /usr/include/alloca.h:35:23: note: expanded from macro 'alloca'
  # define alloca(size)   __builtin_alloca (size)
                          ^
  1 error generated.
  System.Native/CMakeFiles/System.Native.dir/build.make:121: recipe for target 'System.Native/CMakeFiles/System.Native.dir/pal_io.c.o' failed
  make[2]: *** [System.Native/CMakeFiles/System.Native.dir/pal_io.c.o] Error 1
  make[1]: *** [System.Native/CMakeFiles/System.Native.dir/all] Error 2
  CMakeFiles/Makefile2:340: recipe for target 'System.Native/CMakeFiles/System.Native.dir/all' failed
  Makefile:149: recipe for target 'all' failed
  make: *** [all] Error 2
  Failed to build corefx native components.
/home/appveyor/projects/corefx/src/Native/build-native.proj(36,5): error MSB3073: The command ""/home/appveyor/projects/corefx/src/Native/build-native.sh" x64 Debug Linux outconfig netcoreapp-Linux-Debug-x64 -portable" exited with code 1.

dotnet/runtime#33734