libpkgecc: Remove -ffreestanding

This incorporates pkg(8) commit a31c8c3682a3 for consistency.

https://github.com/freebsd/pkg/commit/a31c8c3682a3

MFC after: 1 day
Reviewed by: kevans
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D52920