-rdynamic/--export-dynamic fixes

[squeek502]

• --export-dynamic not getting set via -rdynamic in the ELF linker construction seems to have been a bug? Correct me if I'm wrong, or if -rdynamic should be handled in any of the other linker formats (right now ZigLLVM_MachO sends -export_dynamic and with this PR ZigLLVM_ELF sends --export-dynamic; ZigLLVM_Wasm and ZigLLVM_COFF ignore -rdynamic).
• As far as I can tell, -rdynamic and -Wl,--export-dynamic are essentially the same thing with regards to Zig. The difference in other compilers is that -rdynamic doesn't set --export-dynamic if it doesn't need to for the target? If so, it seems like Zig would probably want to treat the --export-dynamic linker flag the same as -rdynamic (hence why I made the --export-dynamic linker flag set rdynamic = true in zig cc instead of adding a separate export_dynamic field).

Contributes towards #4784

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EDIT:

This is meant to fix warning: unsupported linker arg: --export-dynamic when doing zig cc -Wl,--export-dynamic or zig cc -Wl,-E (or other ways of passing linker args like --for-linker, but those aren't supported by zig cc yet). Full context is here

[daurnimator]

Why -E? for a C compiler it means "preprocessor only". I'm hesitant to overload that meaning.

[squeek502]

-E is an alias for --export-dynamic in ld/lld. Note that this check is only for linker args (i.e. things passed to zig cc via -Wl).

[squeek502]

To be more clear, since I realized I didn't include this in the OP:

This is meant to fix warning: unsupported linker arg: --export-dynamic when doing zig cc -Wl,--export-dynamic or zig cc -Wl,-E (or other ways of passing linker args like --for-linker, but those aren't supported by zig cc yet). Full context is here