shims/super/cc: fix linker flag parsing

[carlocab]

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Our parsing of linker flags can be easily confused by, e.g.,

-Wl,-undefined -Wl,dynamic_lookup,-dead_strip_dylibs

The current code that tries to detect these flags will erroneously
conclude that they were not passed.

This change fixes that.

[Bo98]

-Wl,-undefined -Wl,dynamic_lookup,-dead_strip_dylibs

Weird sequence but technically valid

[carlocab]

I guess there's also a simpler flag that tricks the existing parsing:

-Wl,-undefined,dynamic_lookup,-dead_strip_dylibs

[MikeMcQuaid]

Thanks @carlocab!