selftests/bpf: add -fno-strict-aliasing to BPF_CFLAGS

selftests/bpf: add -fno-strict-aliasing to BPF_CFLAGS

Following the discussion at [1], set -fno-strict-aliasing flag for all
BPF object build rules. Remove now unnecessary <test>-CFLAGS variables.

[1] https://lore.kernel.org/bpf/20250106185447.951609-1-ihor.solodrai@pm.me/

CC: Jose E. Marchesi <jose.marchesi@oracle.com>
Signed-off-by: Ihor Solodrai <ihor.solodrai@pm.me>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250106201728.1219791-1-ihor.solodrai@pm.me
Signed-off-by: Alexei Starovoitov <ast@kernel.org>