parisc: add missing export of __cmpxchg_u8()

__cmpxchg_u8() had been added (initially) for the sake of
drivers/phy/ti/phy-tusb1210.c; the thing is, that drivers is
modular, so we need an export

Fixes: b344d6a83d01 "parisc: add support for cmpxchg on u8 pointers"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>