[tools] Support building shared libLLVM and libClang for SerenityOS

[ADKaster]

Summary

This patch tells CMake that the --whole-archive linker option should be
used for specifying the archives whose members will constitute these
shared libraries.

Symbol versioning is disabled, as the SerenityOS loader doesn't support
it, and the ELF sections that store version data would just waste space.

Depends on D154400

[ADKaster]

prefer CMake platform defined name SERENITYOS over CMAKE_SYSTEM_NAME

llvm/cmake/modules/HandleLLVMOptions.cmake

if(CYGWIN)
     set(LLVM_ON_UNIX 1)
   else(CYGWIN)
     set(LLVM_ON_WIN32 1)
     set(LLVM_ON_UNIX 0)
   endif(CYGWIN)
 elseif(FUCHSIA OR UNIX)
   set(LLVM_ON_WIN32 0)
   set(LLVM_ON_UNIX 1)
-  if(APPLE OR ${CMAKE_SYSTEM_NAME} MATCHES "AIX")
+  if(APPLE OR SERENITYOS OR ${CMAKE_SYSTEM_NAME} MATCHES "AIX")
     set(LLVM_HAVE_LINK_VERSION_SCRIPT 0)
   else()
     set(LLVM_HAVE_LINK_VERSION_SCRIPT 1)
   endif()
 elseif(CMAKE_SYSTEM_NAME STREQUAL "Generic")
   set(LLVM_ON_WIN32 0)
   set(LLVM_ON_UNIX 0)
   set(LLVM_HAVE_LINK_VERSION_SCRIPT 0)

llvm/tools/llvm-shlib/CMakeLists.txt

if(LLVM_BUILD_LLVM_DYLIB)
   list(REMOVE_DUPLICATES LIB_NAMES)
   if(("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux") OR (MINGW) OR (HAIKU)
      OR ("${CMAKE_SYSTEM_NAME}" STREQUAL "FreeBSD")
      OR ("${CMAKE_SYSTEM_NAME}" STREQUAL "GNU")
      OR ("${CMAKE_SYSTEM_NAME}" STREQUAL "OpenBSD")
      OR ("${CMAKE_SYSTEM_NAME}" STREQUAL "Fuchsia")
      OR ("${CMAKE_SYSTEM_NAME}" STREQUAL "DragonFly")
      OR ("${CMAKE_SYSTEM_NAME}" STREQUAL "Android")
+     OR ("${CMAKE_SYSTEM_NAME}" STREQUAL "SerenityOS")
      OR ("${CMAKE_SYSTEM_NAME}" STREQUAL "SunOS")) # FIXME: It should be "GNU ld for elf"
     configure_file(
     ${CMAKE_CURRENT_SOURCE_DIR}/simple_version_script.map.in
     ${LLVM_LIBRARY_DIR}/tools/llvm-shlib/simple_version_script.map)
 
     # GNU ld doesn't resolve symbols in the version script.
     set(LIB_NAMES -Wl,--whole-archive ${LIB_NAMES} -Wl,--no-whole-archive)
-    if (NOT LLVM_LINKER_IS_SOLARISLD AND NOT MINGW)
+    if (NOT LLVM_LINKER_IS_SOLARISLD AND NOT MINGW AND NOT SERENITYOS)
       # Solaris ld does not accept global: *; so there is no way to version *all* global symbols
       set(LIB_NAMES -Wl,--version-script,${LLVM_LIBRARY_DIR}/tools/llvm-shlib/simple_version_script.map ${LIB_NAMES})
     endif()
     if (NOT MINGW)
       # Optimize function calls for default visibility definitions to avoid PLT and
       # reduce dynamic relocations.
       # Note: for -fno-pic default, the address of a function may be different from
       # inside and outside libLLVM.so.