`-flat_namespace` linking issue when cross-compiling GMP-based library for macOS x86_64 on arm64

[Andrej Zhilenkov]

Hello!

Unsure if my issue is part of
https://gmplib.org/list-archives/gmp-bugs/2024-October/005539.html or
it's something else,
but still might be worth reporting (at least traceback will be
googlable and might lead someone to the conclusions quicker).

I've encountered an error trying to cross compile Python extension
that's using GMP on arm64 machine for x86_64 macOS target.
The error looks like this:
```
ld: branch8 out of range 366651 in ___gmpn_add_nc
final section layout:
    __TEXT               addr=0x00000000, size=0x02d09000, fileOffset=0x00000000
        __text           addr=0x00001400, size=0x0264a5e4, fileOffset=0x00001400
        __stubs          addr=0x0264b9e4, size=0x0000423c, fileOffset=0x0264b9e4
        __stub_helper    addr=0x0264fc20, size=0x00006c58, fileOffset=0x0264fc20
        __gcc_except_tab addr=0x02656878, size=0x002efc04, fileOffset=0x02656878
        __const          addr=0x02946480, size=0x001cdc18, fileOffset=0x02946480
        __cstring        addr=0x02b140a0, size=0x0014324b, fileOffset=0x02b140a0
        __unwind_info    addr=0x02c572ec, size=0x0008d2d0, fileOffset=0x02c572ec
        __eh_frame       addr=0x02ce45c0, size=0x00024a30, fileOffset=0x02ce45c0
    __DATA_CONST         addr=0x02d09000, size=0x0022b000, fileOffset=0x02d09000
        __got            addr=0x02d09000, size=0x00001dc8, fileOffset=0x02d09000
        __mod_init_func  addr=0x02d0adc8, size=0x00000690, fileOffset=0x02d0adc8
        __const          addr=0x02d0b460, size=0x00227fd0, fileOffset=0x02d0b460
    __DATA               addr=0x02f34000, size=0x000c3000, fileOffset=0x02f34000
        __la_symbol_ptr  addr=0x02f34000, size=0x000056a0, fileOffset=0x02f34000
        __data           addr=0x02f396a0, size=0x0005f7b8, fileOffset=0x02f396a0
        __thread_vars    addr=0x02f98e58, size=0x000006d8, fileOffset=0x02f98e58
        __thread_ptrs    addr=0x02f99530, size=0x00000178, fileOffset=0x02f99530
        __thread_data    addr=0x02f996b0, size=0x000000d8, fileOffset=0x02f996b0
        __thread_bss     addr=0x02f99790, size=0x00018850, fileOffset=0x02f99790
        __bss            addr=0x02fb1fe0, size=0x0003b0c0, fileOffset=0x00000000
        __common         addr=0x02fed0a0, size=0x00009098, fileOffset=0x00000000
    __LINKEDIT           addr=0x02ff7000, size=0x0148e000, fileOffset=0x02fb2000
clang: error: linker command failed with exit code 1 (use -v to see invocation)
```

Investigating and trying to narrow it down, I've found that the issue
is unrelated to Python itself -
it can be reproduced by compiling any library that uses GMP and using
the `-flat-namespace` flag during the linking.

I've created a repo that reproduced the issue using Github Workflow.
See - https://github.com/Andrej730/test-gmp-mac-cross-compilation/actions/runs/18943199817/job/54087191838
Backup pastebin - https://pastebin.com/pB0SntM0

I'm not exactly sure if I do need the `-flat-namespace` flag in my
case - we were using `-Wl,-flat_namespace,-undefined,suppress` for
ages now when compiling that Python extension and maybe it's not
needed anymore.
Reading through previous thread I've found that `-flat-namespace` and
`-undefined suppress` are somehow connected historically and since
`-undefined suppress` is officially deprecated (clang prints a warning
when it's used), then maybe switching to `-undefined dynamic_lookup`
will do the trick without `-flat-namespace`.
So I'll try to disable it for now as a workaround for this issue.

Any thoughts on this? Is it the same issue as in the thread linked
above and perhaps the issue with `-flat_namespace` can be
noted either on "Known Build Problems" / "Notes for Particular
System"? Or is it something else worth investigating?

Thanks for your time and for maintaining GMP.