audio/flac: fix build on powerpc64

FreeBSD doesn't support getauxval(), it has elf_aux_info() instead. Use this to fix build on powerpc64.
Disable VSX to not use VSX-related functions.

PR: 239870
Approved by: linimon (mentor), naddy (maintainer)
Differential Revision: https://reviews.freebsd.org/D21284