cgen: #undef some more predefined symbols. (#5831)
cgen: #undef some more predefined symbols. (#5831)

This is a followup to issue #5171.
tulayang/Nim,russpowers/Nim,tulayang/Nim,russpowers/Nim,tulayang/Nim,dom96/Nim,russpowers/Nim,russpowers/Nim,dom96/Nim,tulayang/Nim,tulayang/Nim,russpowers/Nim,russpowers/Nim,russpowers/Nim,tulayang/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim