Re-enable use of LLD when building with LTO (#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 #8204 that removed the usage of lld along
with LTO, but keeps the part that enables it for non-LTO cases.

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0417936..240426c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -219,7 +219,8 @@
   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(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")