Auto merge of #1393 - Susurrus:ptrace_detach, r=gnzlbg

Re-add PTRACE_DETACH for mips64 GNU targets

During the refactor in 4bd419eb, `PTRACE_DETACH` was accidentally removed on
these targets. This re-adds it.

Fixes brokenness encountered during nix-rust/nix#1035

CC @asomers