Use `ucontext_t` instead of `ucontext`

Use `ucontext_t` instead of `ucontext`

Forwarded: not-needed

Both Bionic and glibc define a struct called `ucontext_t` but only Bionic
defines an alias `ucontext_t`. Let's use the more standardized one.

Gbp-Pq: Topic art
Gbp-Pq: Name ucontext.patch