selftests/nolibc: add support for LLVM= parameter

selftests/nolibc: add support for LLVM= parameter

Makefile.include can modify CC and CFLAGS for usage with clang.
Make use of it.

Makefile.include is currently used to handle the O= variable.
This is incompatible with the LLVM= handling as for O= it has to be
included as early as possible, while for LLVM= it needs to be included
after CFLAGS are set up.

To avoid this incompatibility, switch the O= handling to custom logic.

Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20240807-nolibc-llvm-v2-10-c20f2f5fc7c2@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>