randstruct: Split randstruct Makefile and CFLAGS

To enable the new Clang randstruct implementation[1], move
randstruct into its own Makefile and split the CFLAGS from
GCC_PLUGINS_CFLAGS into RANDSTRUCT_CFLAGS.

[1] https://reviews.llvm.org/D121556

Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
7 files changed