selftests/bpf: Attempt to build BPF programs with -Wsign-compare

GCC's -Wall includes -Wsign-compare while clang does not.
Since BPF programs are built with clang we need to add this flag explicitly
to catch problematic comparisons like:

  int i = -1;
  unsigned int j = 1;
  if (i < j) // this is false.

  long i = -1;
  unsigned int j = 1;
  if (i < j) // this is true.

C standard for reference:

- If either operand is unsigned long the other shall be converted to unsigned long.

- Otherwise, if one operand is a long int and the other unsigned int, then if a
long int can represent all the values of an unsigned int, the unsigned int
shall be converted to a long int; otherwise both operands shall be converted to
unsigned long int.

- Otherwise, if either operand is long, the other shall be converted to long.

- Otherwise, if either operand is unsigned, the other shall be converted to unsigned.

Unfortunately clang's -Wsign-compare is very noisy.
It complains about (s32)a == (u32)b which is safe and doen't have surprising behavior.

This patch fixes some of the issues. It needs a follow up to fix the rest.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/bpf/20231226191148.48536-2-alexei.starovoitov@gmail.com