Re-enable use of LLD when building with LTO (WebAssembly#8227)

The Chromium CI builders (which build the emsdk release builds) don't
support LTO builds with the system linker on Mac (but they do have
lld). This reverts the part of WebAssembly#8204 that removed the usage of lld along
with LTO, but keeps the part that enables it for non-LTO cases.

Author: dschuff

CMakeLists.txt

@@ -219,7 +219,8 @@ if(NOT MSVC AND NOT EMSCRIPTEN)
   endif()
 endif()
 
-# The version of clang currently on our Mac bots doesn't seem to support this flag.
+# The version of clang currently on Binaryen's Mac CI doesn't seem to support
+# this flag, so don't enable it by default. LTO also enables lld separately.
 if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT APPLE)
   add_link_flag("-fuse-ld=lld")
 endif()
@@ -228,6 +229,7 @@ if(BYN_ENABLE_LTO)
   if(NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
     message(FATAL_ERROR "ThinLTO is only supported by clang")
   endif()
+  add_link_flag("-fuse-ld=lld")
   set_property(GLOBAL APPEND PROPERTY JOB_POOLS link_job_pool=2)
   set(CMAKE_JOB_POOL_LINK link_job_pool)
   add_compile_flag("-flto=thin")