Use posix_spawn on musl targets
Use posix_spawn on musl targets

The posix_spawn had been available in a form suitable for use in a
Command implementation since musl 0.9.12. Use it in a preference to a
fork when possible, to benefit from CLONE_VM|CLONE_VFORK used there.

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust