pmbootstrap aportgen gcc{,4,6}*: fix depends

Explicitly depend on mpc1, mpfr3. Our generated gcc aports use the !tracedeps option, so we need to explicitly set the libraries it depends on.

This has mostly not been an issue, as we are installing our gcc packages together with Alpine's gcc package, which causes the libraries to get installed anyway.

Related: pmaports#236 (closed)

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After this, we need to bump the pkgrel of main/gcc{4,6}, fix the depends there, and run pmbootstrap aportgen for the existing gcc4,6 aports.