bpf: Use -Wno-error in certain tests when building with GCC

Certain BPF selftests contain code that, albeit being legal C, trigger
warnings in GCC that cannot be disabled.  This is the case for example
for the tests

  progs/btf_dump_test_case_bitfields.c
  progs/btf_dump_test_case_namespacing.c
  progs/btf_dump_test_case_packing.c
  progs/btf_dump_test_case_padding.c
  progs/btf_dump_test_case_syntax.c

which contain struct type declarations inside function parameter
lists.  This is problematic, because:

- The BPF selftests are built with -Werror.

- The Clang and GCC compilers sometimes differ when it comes to handle
  warnings.  in the handling of warnings.  One compiler may emit
  warnings for code that the other compiles compiles silently, and one
  compiler may offer the possibility to disable certain warnings, while
  the other doesn't.

In order to overcome this problem, this patch modifies the
tools/testing/selftests/bpf/Makefile in order to:

1. Enable the possibility of specifing per-source-file extra CFLAGS.
   This is done by defining a make variable like:

   <source-filename>-CFLAGS := <whateverflags>

   And then modifying the proper Make rule in order to use these flags
   when compiling <source-filename>.

2. Use the mechanism above to add -Wno-error to CFLAGS for the
   following selftests:

   progs/btf_dump_test_case_bitfields.c
   progs/btf_dump_test_case_namespacing.c
   progs/btf_dump_test_case_packing.c
   progs/btf_dump_test_case_padding.c
   progs/btf_dump_test_case_syntax.c

   Note the corresponding -CFLAGS variables for these files are
   defined only if the selftests are being built with GCC.

Note that, while compiler pragmas can generally be used to disable
particular warnings per file, this 1) is only possible for warning
that actually can be disabled in the command line, i.e. that have
-Wno-FOO options, and 2) doesn't apply to -Wno-error.

Tested in bpf-next master branch.
No regressions.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240127100702.21549-1-jose.marchesi@oracle.com