Replace 'struct ucontext' with 'ucontext_t' type

Replace 'struct ucontext' with 'ucontext_t' type

glibc used to have:

   typedef struct ucontext { ... } ucontext_t;

glibc now has:

   typedef struct ucontext_t { ... } ucontext_t;

(See https://sourceware.org/bugzilla/show_bug.cgi?id=21457
 for detail and rationale for the glibc change)

However, QEMU used "struct ucontext" in declarations. This is a
private name and compatibility cannot be guaranteed. Switch to
only using the standardized type name.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Message-id: 20170628204452.41230-1-raj.khem@gmail.com
Cc: Kamil Rytarowski <kamil@netbsd.org>
Cc: Riku Voipio <riku.voipio@iki.fi>
Cc: Laurent Vivier <laurent@vivier.eu>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[PMM: Rewrote commit message, based mostly on the one from
 Nathaniel McCallum]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>