bc: open-code bc_parse_updateFunc() macro

bc: open-code bc_parse_updateFunc() macro

This clearly shows one place which would need changing
for nested funcdefs to work.

function                                             old     new   delta
bc_parse_reset                                       106     110      +4
zbc_vm_process                                       586     585      -1
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/1 up/down: 4/-1)                Total: 3 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>