bpf: Tell bpf programs kernel's PAGE_SIZE

vmlinux BTF includes all kernel enums.
Add __PAGE_SIZE = PAGE_SIZE enum, so that bpf programs
that include vmlinux.h can easily access it.

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20240307031228.42896-7-alexei.starovoitov@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>