x86: Add missing prototypes and mark more local functions as static

To be able to compile with -Wmissing-prototypes, we also need prototypes
for functions that are called from assembler code. We put the prototypes
into the .c files and not into header files here, since these functions
are not called from other .c files.
While we're at it, also mark some more functions as static in these
files which are only used locally.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1538123582-17442-3-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>