bpf: Move bpf_jit_get_prog_name() to core.c

bpf_jit_get_prog_name() will be used by all JITs when enabling support
for private stack. This function is currently implemented in the x86
JIT.

Move the function to core.c so that other JITs can easily use it in
their implementation of private stack.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20250724120257.7299-2-puranjay@kernel.org