This updates the generated stdlib.h and malloc.h headers to
include the subsets of extenion functions declared by glibc that
are also supported by Scudo and that use only simple types.
Scudo's extensions not declared by glibc are omitted. glibc's
extensions not implemented by Scudo are omitted. The mallinfo
and mallinfo2 functions are omitted (at least for now) since they
need struct definitions for their return types.
This uses the new merge_yaml_files feature in hdrgen to share the
source of truth for the malloc suite of functions declared in
both stdlib.h and in malloc.h (without either header including
the other). It also modernizes the malloc.yaml definition a bit,
including dropping the custom template malloc.h.def file in favor
of using the explicit macros list to generate the includes.
