bpf: Abstract loop unrolling pragmas in BPF selftests

[Changes from V1:
- Avoid conflict by rebasing with latest master.]

Some BPF tests use loop unrolling compiler pragmas that are clang
specific and not supported by GCC. These pragmas, along with their
GCC equivalences are:

#pragma clang loop unroll_count(N)
#pragma GCC unroll N

#pragma clang loop unroll(full)
#pragma GCC unroll 65534

#pragma clang loop unroll(disable)
#pragma GCC unroll 1

#pragma unroll [aka #pragma clang loop unroll(enable)]
There is no GCC equivalence to this pragma. It enables unrolling on
loops that the compiler would not ordinarily unroll even with
-O2|-funroll-loops, but it is not equivalent to full unrolling
either.

This patch adds a new header progs/bpf_compiler.h that defines the
following macros, which correspond to each pair of compiler-specific
pragmas above:

__pragma_loop_unroll_count(N)
__pragma_loop_unroll_full
__pragma_loop_no_unroll
__pragma_loop_unroll

The selftests using loop unrolling pragmas are then changed to include
the header and use these macros in place of the explicit pragmas.

Tested in bpf-next master.
No regressions.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20240208203612.29611-1-jose.marchesi@oracle.com

selftests: Use consistent include paths for libbpf

Fix all selftests to include libbpf header files with the bpf/ prefix, to
be consistent with external users of the library. Also ensure that all
includes of exported libbpf header files (those that are exported on 'make
install' of the library) use bracketed includes instead of quoted.

To not break the build, keep the old include path until everything has been
changed to the new one; a subsequent patch will remove that.

Fixes: 6910d7d3867a ("selftests/bpf: Ensure bpf_helper_defs.h are taken from selftests dir")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/157952560568.1683545.9649335788846513446.stgit@toke.dk

selftests/bpf: add loop test 4

Add a test that returns a 'random' number between [0, 2^20)
If state pruning is not working correctly for loop body the number of
processed insns will be 2^20 * num_of_insns_in_loop_body and the program
will be rejected.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>