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

[drodriguez]

Summary

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.

[smeenai]

LGTM

We should fix llvm-strip to accept grouped flags, but the other side of this branch was already using non-grouped flags anyway :)

[drodriguez]

https://reviews.llvm.org/D105249 adds the support for grouped options. I will keep the options separated in this commit (if nobody has any concerns), since it seems more portable at this point.

llvm/cmake/modules/AddLLVM.cmake

if(NOT LLVM_EXTERNALIZE_DEBUGINFO)
     return()
   endif()
 
   if(NOT LLVM_EXTERNALIZE_DEBUGINFO_SKIP_STRIP)
     if(APPLE)
       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()
   endif()
 
   if(APPLE)
     if(LLVM_EXTERNALIZE_DEBUGINFO_EXTENSION)
       set(file_ext ${LLVM_EXTERNALIZE_DEBUGINFO_EXTENSION})