selftests/bpf: Use program name for skel load/destroy functions

selftests/bpf: Use program name for skel load/destroy functions

In preparation to migrate tests from bpf/test_sock_addr.c to
sock_addr.c, update BPF_SKEL_FUNCS so that it generates functions
based on prog_name instead of skel_name. This allows us to differentiate
between programs in the same skeleton.

Signed-off-by: Jordan Rife <jrife@google.com>
Link: https://lore.kernel.org/r/20240510190246.3247730-3-jrife@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>