x86: Declare local functions as "static" and specify argument types

This makes the code compilable with the compiler flags -Wstrict-prototypes
and -Wmissing-prototypes.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1538123582-17442-2-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>