Add -fPIC to the arm64 cflags where needed

[andrew]

Summary

This adds -fPIC to CFLAGS_aarch64 to a number of ports that already have it
in the amd64 CFLAGS, and are failing to build.

Test Plan

This is untested and should have an aarch64 Poudriere run.

[emaste]

Should be fine to turn on -- these ports don't build on FreeBSD/arm64 today so we don't have arm64 regressions to worry about here :)