Use the POSIX types for ucontext_t/mcontext_t.

Noticed while trying to copy & paste arm64 code for riscv64 (which had
a different name for the implementation-detail `struct`, but the POSIX
`typedef` would have worked fine).

Test: treehugger
Change-Id: I4880fdc821e564aafecf19f880b6c3ba4c13b072
6 files changed