libbpf: Fix libbpf build on compilers missing __builtin_mul_overflow

GCC compilers older than version 5 don't support __builtin_mul_overflow yet.
Given GCC 4.9 is the minimal supported compiler for building kernel and the
fact that libbpf is a dependency of resolve_btfids, which is dependency of
CONFIG_DEBUG_INFO_BTF=y, this needs to be handled. This patch fixes the issue
by falling back to slower detection of integer overflow in such cases.

Fixes: 029258d7b228 ("libbpf: Remove any use of reallocarray() in libbpf")
Signed-off-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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