I was confused at first why tree-core.h was undefining DEF_BUILTIN_CHKP
before defining it, then undefining it again after including builtins.def.
This is because builtins.def provides a default definition of
DEF_BUILTIN_CHKP, but leaves it up to the caller to undefine it where
necessary. Similarly to the previous internal-fn.def patch, it seems
more obvious for builtins.def to #undef things unconditionally.
One argument might have been that keeping preprocessor stuff
out of the .def files makes it easier for non-cpp parsers. In practice
though we already have #ifs and multiline #defines, so single-line #undefs
should be easy in comparison.
Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.