alpha: Restore symbol versions for symbols exported from assembly

Add <asm/asm-prototypes.h> so that genksyms knows the types of
these symbols and can generate CRCs for them.

Fixes: 00fc0e0d

 ("alpha: move exports to actual definitions")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Matt Turner <mattst88@gmail.com>