lto.inc: add clang specific LTO configuration

lto.inc: add clang specific LTO configuration

This commit adds support for Clang-specific LTO options.

1. -fuse-linker-plugin is not supported by Clang.
2. -ffat-lto-objects is used to ensure that object files (.o) are generated as ELF
   even when LTO is applied. During the dwarfsrcfiles process, if LTO is used
   with the Clang toolchain, static library object files are skipped if they are
   LLVM bitcode instead of ELF. Therefore, it is recommended to add this to
   LTOEXTRA if necessary.
3. Adding 'thin-lto' to DISTRO_FEATURES enables -flto=thin.

Signed-off-by: mark.yang <mark.yang@lge.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>