The guest image reader and the module builder already produce a
platform-neutral GuestImage; --macos adds a third writer next to the ELF
and PE ones. The executable and every bundled module become x86-64 Mach-O
files (modules as dylibs in app0/sce_module) that run under Rosetta on
Apple silicon.
FreeBSD code reaches its thread pointer through %fs, which macOS cannot
set, so the writer rewrites each %fs access into a stub that reads the
image's thread pointer from a pthread key and allocates the TLS block on
a thread's first access. Dynamic TLS goes through the image's descriptor
instead of __tls_get_addr. Imports bind by NID to the prx libraries
beside the output, exports and TLS imports use the dynamic symbol names,
and the image's metadata lives in __APS5DATA,__meta.
--macos conflicts with --windows. The macOS runtime that consumes the
output (TLS allocation, libraries built for macOS) comes in separate
changes.
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