golang.org/x/sys/unix fails to build on aarch64 because the pinned vendored
FpReg references an undefined uint128 type, at least on FreeBSD arm64:
vendor/golang.org/x/sys/unix/ztypes_freebsd_arm64.go:400:12: undefined: uint128
Fake it with an equivalent 16x byte array, which is what upstream now does: