x86/purgatory: Make functions and variables static

Sparse emits several 'symbol not declared' warnings for various
functions and variables.

Add static keyword to functions and variables which have file scope
only.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Link: http://lkml.kernel.org/r/1487545956-2547-2-git-send-email-me@tobin.cc


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>