Avoid calling libc in druntime array append function

[ryuukk]

Context:

ldc-developers/ldc#4324

I don't know if that's the right place to do that, but i don't know what should be done, and none cared apparently when templating everything

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[dkorpel]

Superseded by #14912 (see ldc-developers/ldc#4324 (comment))

[ryuukk]

I think this PR is still valid, if we can reduce reliance on imports from libc, we move towards an easier to port runtime

[dkorpel]

memcpy has to be defined regardless, LDC's backend sometimes emits calls to it. Rolling an own implementation is also likely a performance regression, since memcpy is performance critical and optimized a lot.

[ryuukk]

memcpy has to be defined regardless, LDC's backend sometimes emits calls to it. Rolling an own implementation is also likely a performance regression, since memcpy is performance critical and optimized a lot.

compilers will optimize this anyways