libbpf: Minor style improvements in USDT code

Fix several typos and references to non-existing headers.
Also use __BYTE_ORDER__ instead of __BYTE_ORDER for consistency with
the rest of the bpf code - see commit 45f2bebc ("libbpf: Fix
endianness detection in BPF_CORE_READ_BITFIELD_PROBED()") for
rationale).

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220407214411.257260-2-iii@linux.ibm.com