FreeBSD: make wchar_t unsigned on arm/aarch64

src/unix/bsd/apple/mod.rs

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

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

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

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

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

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

src/unix/bsd/mod.rs

src/unix/bsd/netbsdlike/mod.rs