Auto merge of #1554 - nmattia:nm-mcontext, r=gnzlbg
Auto merge of #1554 - nmattia:nm-mcontext, r=gnzlbg

Refine definition of mcontext_t on x86_64-unknown-linux-musl

Fixes #1553

I've inlined the links I used for defining constants and fields, although they pretty much map to the changes introduced in ac7f0fac2f34d4eed1f05e9d20f236149711ff87 and ae06c2b2ae6938c9162b9deadae442194a816ad2.

rust-lang/libc,rust-lang/libc,rust-lang/libc