x86, um: sanitize uml sigcontext.h uses

a) the only difference between sigcontext and sysdep/sigcontext
is that the former contains externs for two long-dead functions.
Removed, switched the only user to sysdep/sigcontext

b) asm/sigcontext.h is removable - that of underlying architecture
would get used.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>