sysutils/paladin: fix build on riscv64

Bump golang.org/x/sys dependency to a version new enough
to support riscv64-freebsd. This subsumes the patches
previously included for AArch64 support.

While we are at it, fix configuration when PREFIX is not
/usr/local.

Sponsored by: Berliner Linux User Group e.V.
Approved by: portmgr (build fix blanket)