[compiler-rt] Build crtbegin.o/crtend.o for SerenityOS

[ADKaster]

Summary

Depends on D154396

[ADKaster]

Also patch builtin-config-ix.cmake

[MaskRay]

This looks good and can land separately. While some projects may want to land everything together, in llvm-project it's not bad to land some patches separately.
As there is a risk that the first patch of series gets reverted due to breakage to some systems and then we may need to revert the dependent patches...
I can land this one for you if you don't have write access to the repository.

[ADKaster]

I don't personally have write access, but the original author of the patches, @BertalanD does. For all the patches except the profiling one, he should be the commit author. The other patches have Co-Authored-By lines in the Diff description (and on my fork https://github.com/ADKaster/llvm-project/commits/serenityos-upstream) as appropriate from our refinement out-of-tree.

compiler-rt/cmake/builtin-config-ix.cmake

else()
   if(COMPILER_RT_BUILTINS_STANDALONE_BUILD)
     test_targets()
   endif()
   # Architectures supported by compiler-rt libraries.
   filter_available_targets(BUILTIN_SUPPORTED_ARCH
     ${ALL_BUILTIN_SUPPORTED_ARCH})
 endif()
 
-if (OS_NAME MATCHES "Linux" AND NOT LLVM_USE_SANITIZER)
+if (OS_NAME MATCHES "Linux|SerenityOS" AND NOT LLVM_USE_SANITIZER)
   set(COMPILER_RT_HAS_CRT TRUE)
 else()
   set(COMPILER_RT_HAS_CRT FALSE)
 endif()
 
 message(STATUS "Builtin supported architectures: ${BUILTIN_SUPPORTED_ARCH}")

compiler-rt/cmake/crt-config-ix.cmake

if(NOT APPLE)
   if(COMPILER_RT_CRT_STANDALONE_BUILD)
     test_targets()
   endif()
   # Architectures supported by compiler-rt crt library.
   filter_available_targets(CRT_SUPPORTED_ARCH ${ALL_CRT_SUPPORTED_ARCH})
   message(STATUS "Supported architectures for crt: ${CRT_SUPPORTED_ARCH}")
 endif()
 
-if (CRT_SUPPORTED_ARCH AND OS_NAME MATCHES "Linux" AND NOT LLVM_USE_SANITIZER)
+if (CRT_SUPPORTED_ARCH AND OS_NAME MATCHES "Linux|SerenityOS" AND NOT LLVM_USE_SANITIZER)
   set(COMPILER_RT_HAS_CRT TRUE)
 else()
   set(COMPILER_RT_HAS_CRT FALSE)
 endif()