Fix illumos native build

[am11]

Three errors were reported by illumos crossbuild (gcc8 on linux targetting illumos); one each in coreclr, installer and libraries.

1. Missing header guard for PCH (not sure why it does not happen in CI running gcc7 for native linux compile?):

  [ 56%] Building CXX object src/ToolBox/superpmi/superpmi/CMakeFiles/superpmi.dir/commandline.cpp.o
  In file included from /runtime/src/coreclr/src/ToolBox/superpmi/superpmi/commandline.cpp:10:
  /runtime/src/coreclr/src/ToolBox/superpmi/superpmi/../superpmi-shared/standardpch.h:114:20: error: redefinition of 'void __debugbreak()'
   static inline void __debugbreak()
                      ^~~~~~~~~~~~
  In file included from /runtime/artifacts/obj/coreclr/illumos.x64.Release/src/ToolBox/superpmi/superpmi/CMakeFiles/superpmi.dir/cmake_pch.hxx:5,
                   from <command-line>:
  /runtime/src/coreclr/src/ToolBox/superpmi/superpmi/../superpmi-shared/standardpch.h:114:20: note: 'void __debugbreak()' previously defined here
   static inline void __debugbreak()
                      ^~~~~~~~~~~~
  src/ToolBox/superpmi/superpmi/CMakeFiles/superpmi.dir/build.make:97: recipe for target 'src/ToolBox/superpmi/superpmi/CMakeFiles/superpmi.dir/commandline.cpp.o' failed
  CMakeFiles/Makefile2:4320: recipe for target 'src/ToolBox/superpmi/superpmi/CMakeFiles/superpmi.dir/all' failed
  make[2]: *** [src/ToolBox/superpmi/superpmi/CMakeFiles/superpmi.dir/commandline.cpp.o] Error 1
  make[1]: *** [src/ToolBox/superpmi/superpmi/CMakeFiles/superpmi.dir/all] Error 2
  make[1]: *** Waiting for unfinished jobs....

2. System.Globalization with ICU is now building as part of libraries (previously it was in coreclr), so it needed HAVE_SET_MAX_VARIABLE from coreclr's tryrun.cmake to the one under libraries.

3. Corehost's static plan requires -lsocket when linking libcoreclr.

cc @janvorli

[janvorli]

LGTM, thank you!

[janvorli]

The crash in System.Runtime.Serialization.Formatters.Tests is #44250