char is unsigned on FreeBSD aarch64 and powerpc64

src/unix/bsd/freebsdlike/dragonfly/mod.rs

src/unix/bsd/freebsdlike/freebsd/aarch64.rs

src/unix/bsd/freebsdlike/freebsd/powerpc64.rs

src/unix/bsd/freebsdlike/freebsd/x86.rs

src/unix/bsd/freebsdlike/freebsd/x86_64.rs

src/unix/bsd/freebsdlike/mod.rs