Missing bpf_cpu_to_be64 for follow-up commit.

Signed-off-by: Timo Beckers <timo@isovalent.com>

This patch adds support for 64-bit wide ELF substitutions, to be used in
a follow-up patch.

Signed-off-by: Timo Beckers <timo@isovalent.com>

Historically, the largest data type used for declaring global data was uint32.
Cilium's iproute2 fork had patches that made the assumption all values are
32 bits, so this behaviour was replicated when it was ported to Go. This only
works well if the largest variable emitted to .data is 32 bits wide. Any
smaller and the section's alignment would drop, causing the inliner to read
part of the next variable. Any larger and only part of a u64 would be read.

Since we now no longer rely on iproute2 for loading any ELFs, we can start
changing the way this works. In Go, we have access to .data's BTF Datasec,
so we can be a bit smarter about it and detect how wide each variable is,
so we can accurately copy out the bits needed.

This patch introduces a uint64 global data macro, DEFINE_U64, and updates
DEFINE_IPV6 to use it. This cuts down on the amount of instructions needed
to carry ipv6 values in the instruction stream from 8 to 4.

Signed-off-by: Timo Beckers <timo@isovalent.com>