tools/nolibc: compiler: use attribute((naked)) if available

The current entrypoint attributes optimize("Os", "omit-frame-pointer")
are intended to avoid all compiler generated code, like function
porologue and epilogue.
This is the exact usecase implemented by the attribute "naked".

Unfortunately this is not implemented by GCC for all targets,
so only use it where available.
This also provides compatibility with clang, which recognizes the
"naked" attribute but not the previously used attribute "optimized".

Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20240807-nolibc-llvm-v2-6-c20f2f5fc7c2@weissschuh.net


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>