remove obsolete -m64 option

[tschuett]

Apple ld man page

 -m          Don't treat multiple definitions as an error.  This is no longer supported. This option is obsolete.

LLVM MachO lld: option is obsolete
https://github.com/llvm/llvm-project/blob/70804f2a2f7b87227a873cd6d8852ad295068e79/lld/MachO/Options.td#L1040

The LLVM lld is not going to implement -m64. It prevents bringing LLVM lld MachO support for rust.

see #85938

[rust-highfive]

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