Build failure on macOS Sonoma 14.0 beta

[Marc Culler]

This message is just to make sure that the situation is clear.:

1.  Building gmp with -enable-fat fails with the XCode 15 compiler and the
new XCode 15 linker. The error reported by the linker is impossible.  It is
reporting an out of range branch instruction in a function which is written
in assembly language and which visibly contains no branch instructions
whatsoever.

2. The build succeeds when using the same compiler, but the XCode 14
linker, enabled by adding -ld64 as a linker option.

So the same collection of object files produces an impossible linker error
with the new linker and no error with the old linker.  I consider that
proof that the problem is caused by a bug in the new Apple linker.

No, of course the GMP project is not responsible for Apple bugs.  But this
Apple bug will affect the project because some people will try to build GMP
with the -enable-fat option using XCode 15 and those people will encounter
the same bug.  A small percentage of them will report the bug to you.  So I
think that it will be helpful to your project to be aware of the bug, even
if you are not responsible for it.

- Marc