Change weak target family descriptors to references

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This change replaces the default weak family descriptor structs (each taking up the full family descriptor size) in target_family.c with weak references. The purpose is to save .rodata space.

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[0xc0170]

Can you check test failures to proceed here?

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[mathias-arm]

I think this PR has gotten stale. A lot has been merged to develop, but changing

__attribute__((weak)) const target_family_descriptor_t g_xxxxx  = {0};

to

extern __WEAK const target_family_descriptor_t g_xxxx;

caused some issue with armcc so it was backed out (see [fb4b2292981671c6441d08eec8e51c3a685f4390]).