 Here is the Python code that uses the bcc library to create a histogram of system-wide strlen return values, handling the case where strlen is an indirect function:

```python
from bcc import BPF, Usage
from collections import defaultdict
import time
import signal

class StrlenHistogram(BPF):
    def __init__(self):
        self.hist = defaultdict(int)
        self.strlen_func = None
        self.strlen_impl_offset = 0

    @staticmethod
    def strlen_counter_prog_src():
        return """
        #include <uapi/linux/bpf.h>
        #include <uapi/linux/ptrace.h>
        #include <uapi/linux/bpf_helpers.h>

        SEC("kprobe/strlen")
        int BPF_KPROBE(strlen, struct pt_regs *ctx) {
            u64 impl_addr = bpf_get_current_func_addr_kprobe();
            u32 impl_offset = (u32)(impl_addr - (u64)strlen_impl);
            u32 key = (u32)impl_offset;
            u64 value = bpf_get_stack_kprobe_arg(ctx, 0);
            bpf_map_update_elem(ctx, &strlen_histogram, &value, &key, BPF_ANY);
            return 0;
        }
        """

    @staticmethod
    def strlen_impl_prog_src():
        return """
        #include <uapi/linux/bpf.h>
        #include <uapi/linux/ptrace.h>
        #include <uapi/linux/bpf_helpers.h>

        SEC("kprobe/strlen_impl")
        int BPF_KPROBE(strlen_impl, struct pt_regs *ctx) {
            return 0;
        }
        """

    def attach(self):
        self.strlen_func = self.find_library("libc").find_symbol("strlen")
        self.strlen_impl = self.find_library("libc").find_function("strlen_impl")
        self.strlen_impl_offset = self.find_offset(self.strlen_impl)

        self.strlen_counter_prog = self.load_bpf_code(self.strlen_counter_prog_src())
        self.strlen_impl_prog = self.load_bpf_code(self.strlen_impl_prog_src())

        self.strlen_counter_prog.attach_kprobe(event="strlen", fn_name=self.strlen_func)
        self.strlen_impl_prog.attach_kprobe(event="strlen_impl", fn_name=self.strlen_impl)

    def find_offset(self, function):
        user_functions, user_addresses = self.get_user_functions_and_addresses()
        for func, addr in user_functions:
            if func == function:
                return addr - self.get_bpf_fd()

    def run(self):
        self.attach()
        hist = self.hist
        while True:
            self.items_lookup_and_delete_batch(hist)
            time.sleep(1)
            print(hist)

def main():
    hist = StrlenHistogram()
    signal.signal(signal.SIGINT, hist.handle_loop)
    hist.run()

if __name__ == "__main__":
    main()
```

This code defines a `StrlenHistogram` class that initializes a histogram, finds the symbols for strlen and strlen_impl, and attaches the BPF programs to the appropriate functions. The `run` method updates the histogram, prints it, and clears it every second. The main function sets up a signal handler for SIGINT and starts the histogram's run method.