[runtimes] Don't link against compiler-rt when we don't find it

Otherwise, we would end up passing `-lNOTFOUND` to the compiler, which
caused various compiler checks to fail and ended up breaking the build
in the most obscure ways. For example, checks for -faligned-allocation
would fail because the compiler would complain about an unknown library
called NOTFOUND, and we would end up not passing -faligned-allocation
anywhere in our build. This is madness.

An even better alternative would be to simply FATAL_ERROR if we don't
find the builtins library. However, it seems like our build has been
working fine without finding it for a while, so instead of making a
bunch of builds fail, we can figure out why linking against compiler-rt
doesn't actually seem to be required in a follow-up, and perhaps
relax that.

libcxx/cmake/config-ix.cmake

@@ -61,7 +61,11 @@ if (CXX_SUPPORTS_NOSTDLIBXX_FLAG OR C_SUPPORTS_NODEFAULTLIBS_FLAG)
     include(HandleCompilerRT)
     find_compiler_rt_library(builtins LIBCXX_BUILTINS_LIBRARY
                              FLAGS ${LIBCXX_COMPILE_FLAGS})
-    list(APPEND CMAKE_REQUIRED_LIBRARIES "${LIBCXX_BUILTINS_LIBRARY}")
+    if (LIBCXX_BUILTINS_LIBRARY)
+      list(APPEND CMAKE_REQUIRED_LIBRARIES "${LIBCXX_BUILTINS_LIBRARY}")
+    else()
+      message(WARNING "Could not find builtins library from libc++")
+    endif()
   elseif (LIBCXX_HAS_GCC_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES gcc)
   elseif (LIBCXX_HAS_GCC_S_LIB)

libcxxabi/cmake/config-ix.cmake

@@ -41,7 +41,11 @@ if (CXX_SUPPORTS_NOSTDLIBXX_FLAG OR C_SUPPORTS_NODEFAULTLIBS_FLAG)
     include(HandleCompilerRT)
     find_compiler_rt_library(builtins LIBCXXABI_BUILTINS_LIBRARY
                              FLAGS "${LIBCXXABI_COMPILE_FLAGS}")
-    list(APPEND CMAKE_REQUIRED_LIBRARIES "${LIBCXXABI_BUILTINS_LIBRARY}")
+    if (LIBCXXABI_BUILTINS_LIBRARY)
+      list(APPEND CMAKE_REQUIRED_LIBRARIES "${LIBCXXABI_BUILTINS_LIBRARY}")
+    else()
+      message(WARNING "Could not find builtins library from libc++abi")
+    endif()
   else ()
     if (LIBCXXABI_HAS_GCC_S_LIB)
       list(APPEND CMAKE_REQUIRED_LIBRARIES gcc_s)