patches: Allow including custom malloc.h headers

patches: Allow including custom malloc.h headers

We found this while porting Go runtime. The libgo library had its own malloc.h
header which it tried to include, but because of global include paths it
included the malloc.h of newlib. We workaround this by defining
_INCLUDE_MALLOC_H_ guard before including newlib's malloc.h and including our
own malloc.h next.

Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Charalampos Mainas <charalampos.mainas@neclab.eu>