[cmake] Invoke strip without -l and with non-grouped flags.

`llvm-strip` does not support `-l`. Apple's `strip` supports `-l`, but
it is not documented, and the latest code doesn't seem to do anything
meaningful. From the old source code drops it seems that `-l` was added
around version 795 of cctools and removed before 898. The code around
the flag usage in 795 talks about problems with kext and forcing the
execution of `ld -r`, which seems a behaviour that is not enforceable in
latest versions of cctools.

The `-l` flag was added in https://reviews.llvm.org/D15133 without a lot
of explanation.

Since the flag is not active, removing it should not modify the
behaviour for most people (except if someone is trying to compile LLVM
with a really old version of `strip`).

Additionally, break the invocation into two different flags, since
`llvm-strip` doesn't at the moment support grouped flags, and other
`strip` implementations should work the same no matter if grouped or
not.

Test Plan:

Using `strip` from Xcode 12.5 in Big Sur to strip the same binary (a
simple Hello World), using both `-Sxl` and `-Sx` produces exactly the
same binary.

Repeating the same process with `clang` results also in the same binary.

Reviewed By: smeenai

Differential Revision: https://reviews.llvm.org/D105243

cmake/modules/AddLLVM.cmake

@@ -2007,7 +2007,7 @@ function(llvm_externalize_debuginfo name)
       if(NOT CMAKE_STRIP)
         set(CMAKE_STRIP xcrun strip)
       endif()
-      set(strip_command COMMAND ${CMAKE_STRIP} -Sxl $<TARGET_FILE:${name}>)
+      set(strip_command COMMAND ${CMAKE_STRIP} -S -x $<TARGET_FILE:${name}>)
     else()
       set(strip_command COMMAND ${CMAKE_STRIP} -g -x $<TARGET_FILE:${name}>)
     endif()