Use llvm-{objcopy,strip} when clang lld is used.

* Pass LLVM_{OBJCOPY,STRIP} to makefile rules.
* strip.sh is used only in soong-only mode.
  It will use llvm-strip and  and llvm-objcopy
  when --use-llvm-strip is given.
  Keep flags of strip.sh in alphabetic order.
* Tested build/soong/scripts/build-ndk-prebuilts.sh
  by adding "UseClangLld": true, to soong.variables file.

Bug: 80093681
Test: build/soong/scripts/build-ndk-prebuilts.sh
Change-Id: I612267304eea434c7a33cc086b27b577d5f64094