x86/boot: rename sym_phys() to sym_offs()

x86/boot: rename sym_phys() to sym_offs()

This way macro name better describes its function.
Currently it is used to calculate symbol offset in
relation to the beginning of Xen image mapping.
However, value returned by sym_offs() for a given
symbol is not always equal its physical address.

There is no functional change.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>