Auto merge of #29964 - alexcrichton:update-libc, r=brson
Auto merge of #29964 - alexcrichton:update-libc, r=brson

Includes rust-lang-nursery/libc#66 which should:

Closes #29818
Closes #29956

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