In practice the definition of DEF_INTERNAL_FN is never reused after
including internal-fn.def, so we might as well #undef it there.

This becomes more obvious with a later patch that adds other
DEF_INTERNAL_* directives, such as DEF_INTERNAL_OPTAB_FN.
If the includer doesn't care about the information carried in
these new directives, it can simply leave the macro undefined
and internals.def will provide a definition that forwards to
DEF_INTERNAL_FN.  It doesn't make much sense for includers to have
to #undef macros that are defined by internals.def and it seems overly
complicated to get internals.def to undef macros only in the cases
where it provided a definition.  Instead I went with the approach of
#undeffing all the DEF_INTERNAL_* macros unconditionally.

Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.

gcc/
	* internal-fn.def: #undef DEF_INTERNAL_FN at the end.
	* internal-fn.c: Don't undef it here.
	* tree-core.h: Likewise.

From-SVN: r229923