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The specific attributes for the _start entrypoint are duplicated for
each architecture.
Deduplicate it into a dedicated #define into compiler.h.

For clang compatibility, the epilogue will also need to be adapted, so
move that one, too.
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20240807-nolibc-llvm-v2-5-c20f2f5fc7c2@weissschuh.net

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