[native_toolchain_c] LinkerOptions.treeshake fails to link on macOS and iOS if a symbol to keep is not defined #3737

LinkerOptions.treeshake(symbolsToKeep: ...) passes -Wl,-u,_<symbol> for every symbol to keep on macOS and iOS. ld64 documents -u symbol_name as "specifies that symbol symbol_name must be defined for the link to succeed", so if one of the symbols isn't defined by the input archives, the link fails with an undefined-symbols error.

Tree-shaking can easily ask for symbols that a library doesn't define, for example when bindings cover functions that a particular build or target leaves out. #3726 fixes the equivalent problem on Windows (LNK2001 from the generated module-definition file) by reading the symbol table of the input archives and only keeping the symbols they define. Linux and Android don't have the problem: for shared libraries, -u of a symbol that nothing defines is a no-op there.

Doing the same for macOS and iOS needs a reader for the BSD archive symbol table, which differs from the COFF/GNU one that #3726 reads: the first member is named __.SYMDEF or __.SYMDEF SORTED (or __.SYMDEF_64/__.SYMDEF_64 SORTED for 64-bit tables), usually with a BSD #1/<length> extended name stored at the start of the member data, followed by a little-endian ranlib table of (string offset, member offset) pairs and a string table. Mach-O symbol names carry a leading underscore, so _<symbol> is what to look up.

Until then, packages that tree-shake on macOS and iOS have to filter symbolsToKeep themselves.